Just gotta dust off the ol' microphone and scream into it. (Do it when your parents ain't home so that they don't think you're crazy)
Ok, jokes apart, all you gotta do is know what kind of Pokemon it is and work accordingly in an audio editing software. For example, if my Fakemon is a whale then I'll look up Whale sounds on Google and add some extra flair to changing pitch and reverb and stuff and add some bubble sfx or something, the sky is the limit.
For free audio editing software, I recommend Audacity. You can search many tutorials on how to use it online.
Once you are done with your cry then all you gotta do is save it as a .wav file, move it to Audio/SE/Cries in your game, assign the correct name to it (If the Pokemon is 456 in the National Dex then rename it Cry456.wav) and voila... You have a fakemon cry.