Here's a recipe I've posted a couple times before:
Using a plastic, two liter soda bottle, fill it with water and add half to three quarters of a cup of Saltpeter. Let a natural cord (like cotton clothsline) soak at least an hour (overnight is better), then dry thoroughly.
Outdoors on a dry day with low humidity and sun, it will usually take 3 to 5 hours. The other option is a DRY, warm cellar away from oil burners, etc., this may take two days. If you used too much you will get a thick crusty coating on the match. It will still work but you end up wearing a lot of white residue. Too little or not dry yet will cause you to continually have to relight it. If its crusty wipe the match with a wet rag and let it dry again. It will work fine when dry.
One thing when drying: Avoid iron railing and the like. Saltpeter is a fairly strong oxidizer and will cause rapid rusting of these items even through paint. Galvanized Chain Link fence or clothes line or wood fences or even tree branches work well.