Software: Crossword Compiler, if you're a Windows user. Start with the standard version, but eventually, you'll want the Professional Grid Filler add-on, as it is a great enhancement. You'll also need a better word list than what comes as the default.
In addition to what was mentioned in previous post, there are some useful articles on this (cruciverb) site under the "resources" menu. Some of these are a little dated, but useful all the same.
Study the constructor specs AND the puzzles of publishers you would like to contribute to, so you'll get a feel for what kind of themes they tend to use, their fill and their clues.
Try to make your fill using only ordinary words, familiar phrases, or familiar proper nouns. If you're not sure if it's OK fill, it probably isn't. Check the usage databases (on this site and also crosswordtracker and crosswword giant to see whether a word is likely to be accepted.
Join the Facebook group Crossword Puzzle Collaboration Directory. Lots of helpful folks in that group will answer questions and help you along the way.
