Before you start (this part is kind of long but I promise you that the rest won't take nearly as long)
Now, before you start please note that there are a few types of guns that cannot be silenced without extreme measures or internal modifications so shoot your gun and listen to it first.
Does it sound like an electronic noise (IE something hitting something else, the grinding of gears, or the running of a motor, ect)
if yes then your gun needs a full body silencer, I will not make a tutorial on how to make this without first doing more research on it so make sure than the noise from your gun is the noise of gasses escaping or popping of air release.
One more thing before we start, Do you have access to a tool shed or workshop? There are two types of silencer, one is the Ghetto Silencer which you'll find in most places, the other is a more professional silencer that can actually be used to silence real pistols. The other is much better so if you think you have the tools and materials you may want to follow the half of the instructions labeled "Professional"
(please note none of the pictures taken are of or from me, I just went Via google)