Obviously, the larger the crossword, the more challenging it will be to produce and solve. The program will also be inevitably slower for larger crosswords, so if you do intend to produce large crosswords on a regular basis, you would be advised to have the most powerful computer that you can afford in terms of processing power and system memory. Remember also that by necessity, a large crossword will have to have smaller squares to fit it onto the page!

Normally, a crossword grid has odd-numbered dimensions - e.g. 9x9, 11x11, 13x13, 15x15 etc. This generally works better when filling in black squares for the grid, as this gives a centre line about which outs can be reflected.

Look at the patterns used in magazines and newspapers to see what pattern of outs looks aesthetically pleasing to you. You may wish to copy or emulate the patterns you like, and you can keep them as templates for future crosswords.

The generally accepted ratio for outs to white spaces is 1:6 for American-Style crosswords. This means there should be 1 out for every 6 spaces on the grid. A little up or a little down from this is generally acceptable. Given this ratio, the number of outs for a 15x15 grid would be roughly 37 or 38   (15x15=225/6 =37.5)
The ratio will be much smaller (perhaps about 1:3) for British-style crosswords, as there are far more unkeyed squares.

Generally, outs separate words from each other. English crosswords tend to avoid having 4x4 blocks of outs anywhere in the crossword. American crosswords tend to avoid placing an out such that it would cause any white square to be part of only one clue. A white space that is used for only one clue and not two is known as an unkeyed letter. English crosswords and children's crossword do not tend to enforce the "no unkeyed letters" rule.

Generally, a puzzle looks best, and is more readily accepted, if the outs form a symmetrical pattern about the centre of the grid.

This can be formed by treating the centre column and/or row of the grid as an axis or line, and when drawing an out at one side of the grid, drawing an equivalent out at a position the same distance apart from the centre, at the other side of the grid. In this application, this is known as reflecting.

Thus, horizontal reflection, means that the axis is drawn about the centre column on the grid, so that (for example), a square in the top left hand corner would also appear in the top right hand corner. Similarly, a square 1 row down and to the right from the top left hand corner would also appear in the square 1 row down and to the left from the top right hand corner etc.

Similarly, vertical reflection means that the axis is drawn about the centre row on the grid, so that (for example), a square in the top left hand corner would also appear in the bottom left hand corner. Similarly, a square 1 row down and to the right from the top left hand corner would also appear in the square 1 row up and to the right from the bottom left hand corner etc.

Diagonal reflection is a little more complex, in that the out is reflected about two diagonally drawn axis. Thus, the out appears in the opposite corner of the grid at a position such that if you take the original out, the horizontally reflected out, and the vertically reflected out, the diagonally reflected out would form the fourth corner of a square.

It is best to avoid 2-letter words. It makes your crossword seem unprofessional. If you are forced to have 2-letter words, try to limit the number to no more than four.

If you wish to choose words that are themed in some way, then you are probably aiming for a specific target market. Try to get the approval of a magazine to include such a crossword before you start, as you may waste time producing a crossword with a limited number of publications that might publish it, and find that you cannot use it.

If you are producing a date-limited theme - e.g. a millennium crossword (too late now!), then ensure that you get approval well in advance. Some magazines may be planning six months in ahead, so if you submit your crossword a month before the event, again you will have wasted the effort.

Avoid trademarks or tradenames if possible. Some tradenames have become a common part of English Language - e.g. Formica, Hoover, Sellotape etc. However, some companies fiercely protect use of their trade names, so to avoid costly legal suites, it's best to err on the side of caution - notably, many board games are not happy for you to use their names without prior permission. Also avoid defamatory terms, or terms of a sexual nature.

Avoid abbreviations and chemical symbols where possible.

Generally, it is best to avoid plural words that end in S simply to fit a word into a space easily. Most editors are tolerant of the occasional example of this, but a crossword littered with such plurals looks shoddily constructed.

If you have a rare or strange word, it is best to cross it with fairly easy-to-solve clues, to give the solver the best chance of finding the answer, otherwise you will create much gnashing of teeth and pulling of hair.

Long or unusual words should be placed first on the grid, otherwise you may find yourself having to redo large parts of your crossword. If you do include large words, it is best to alternate vowels and consonants to ensure that word fit easily around them, especially if you are creating an American-style crossword - e.g. DILAPIDATED - you could find an eleven-letter word like this by using the Word List with a mask of @^@^@^@^@^@

Common sources for words and phrases include: WordPuzzle Pro's Clue Pool or Word List, Dictionary, Gazzetteers (for place names), atlases, encyclopaedias, biographical dictionaries, quotations, thesauruses, and other encyclopaedic references.

New Words - i.e. those that don't appear in the dictionary are OK if they are in common use - sometimes dictionaries omit simple derivatives of words - e.g. for the word hard, a dictionary may not include harder or hardest.

Avoid using derivatives of an answer in the clue - e.g. if you give the clue Person who walks for walker, this would be a bit of a giveaway and is one of the most basic taboos in crossword construction. Something like "pedestrian" or "hiker" or cryptically, "Confused elk is pacing in war".

Be careful that your clue does not contain anything that may be deemed as offensive by some. In particular, avoid racist or sexist slurs, or something that might give an opinion on a person or event. If the clue is topical, or about a particular public figure, try to ensure that the clue remains factual. E.g. for "Margaret Thatcher", you might have "Former Female English Prime Minister" which is purely factual, but "Former she-cat-from-hell prime minister who deserved all she got" may be considered inflammatory.

Cryptic clues employ a number of techniques for illustrating the meaning of a word indirectly.

For example, "tome" might be "Large book is not from you" - Large Book is the meaning, the opposite (not) of from you is to me (tome).

Another example might be "A thousand large volumes in a foot digit"  In this case, a thousand corresponds to the Roman Numeral M, and a foot digit is a toe. Thus put M into TOE and you get TOME, which is a large volume or tome.

Another common technique is to use a sound-alike. E.g. "Sounds like foam makes for heavy reading". This means the answer "sounds like foam" - i.e. tome, and a tome is a lengthy book, so makes for heavy reading.

Anagrams are also a popular way of expressing  words - e.g. "An erratic mote makes for heavy reading". In this case, Erratic is the key word to indicate that an anagram follow. Thus an anagram of mote (tome) makes for heavy reading. Common key phrases for anagrams usually derive from something like "jumble, mix, confuse" etc. Sometimes, the key word will follow the anagram, in which case the keyword becomes something like "jumbled, mixed, confused" etc. - e.g. "A mote confused the large volume". Sometimes this context is used also if referring to the subject in the past tense, where it could appear before the subject - e.g. "A confused mote becomes a large volume".

Sometimes it is convenient to add a letter to allow for remaining letters to make an anagram. This would be done by using the first or last letter of a word. E.g. "The first of many within the end of my foot makes for heavy reading". This means that the first letter of many (m) is placed within a word meaning end of my foot - ie. toe, giving TOME.

Letters are sometimes removed from clues to give the answer. E.g. "Confused motet loses its front teeth and becomes a big book" gives a combination of cryptic techniques. Motet loses the first letter of teeth to give MOTE, and confused indicates an anagram of this to give the answer, TOME which is a big book.

Some abbreviations are also used as part of a clue. E.g. "Oh, he sounds like his little trademark needs reforming for his big book". Oh he sounds like the letters OE, and little trademark indicate an abbreviation of trademark, which is TM. Reform indicates an anagram, so take OE and TM and mix them to produce a big book, TOME. Maybe a bit too obscure for most readers!

Of course, you can extend this to various other forms of play on words. You will get the hang of your own style as you go along. E.g. another twist on the TOME might be, "Short Thomas starts exciting book" - Short Thomas is "TOM" start of exciting is "E", TOME is book.

For example, if you wish crosswords to appear in the same colour, or with particular colours for particular types of crosswords, or with specific grid layouts, you can utilise the templates facility to good effect.

If you wish to reuse clues so that solvers get used to particular clues, or if you wish to enforce a particular pre-written style, then you can utilise the clue pool facility to give a standard list of clues, from which everyone can share.