How do you make a silencer?

First, you need to make sure you stay out of prison.

You start by figuring out if they are legal in your state. Don't trust the Word of a friend, or some guy at the gunshop. Read the actual statutes, most states have them online. Look for "silencer" or "suppressor". I only know about Minnesota. They are not legal here.

Once you know you won't go to to state prison for this, you need to make sure you won't go to federal prison. You do this by submitting ATF Form 1, Application to Make and Register a Firearm, to the Bureau of ATF, NFA Branch, along with a check or money order for $200 and getting their approval.

Once this is done, and you have the application with the canceled tax stamp in hand, you can go and make your silencer.

The simplest silencer is a length of pipe with caps and several rubber baffles inside.