Or use Cakewalk for Fruity Reverb 2 if you can't afford to buy FL Studio, Cakewalk is free to use and is compatible with most VSTs, including free ones. You may want to mess around with Cakewalk's own features first though.
You can also use synthentizer or midi-player VSTs and any of Pokémon's soundfont (RBY for Gen 1 styled, GSC for Gen 2, RSE for Gen 3, DPPt for Gen 4, BW or B2W2 for Gen 5, or whatever existing soundfont you love) and make a short "music" with usually two or three instruments. Cries tend to last just one second or just a millisecond, up to two seconds, so try to not make it longer than two seconds, lest it'll stick out like a sore thumb in terms of cry duration compared to official Pokémon.
It's a good idea to mix-and-match instruments for your cries accordingly to fit your different Fakémon. Cross-gen evolutions and babies usually don't have a similar cry to the pre-evo or evo, so don't be afraid to make your fan cross-gen baby or evolution sounding different from the canon Pokémon.
And finally, I'd advice against making it too loud or too soft. If you aren't sure if it would sound too loud or soft for other people, try to compare them to Pokémon's cries uploaded on Youtube. For example, if your custom cries are a lot louder than the uploaded official cries, then you should lower the decibel (db) by lowering the master volume. If too quiet compared to official cries, then raise the master volume instead.
We hope their advices, as well as my own, will help you out a plenty!
