How do you make a silencer for a bb gun?

To manufacture firearms suppressors, you need the appropriate class of Federal Firearms Licence, along with the occupational tax stamp necessary for manufacturing NFA items. Making such a device for a pellet gun is a bit of a thin line to walk - if it's determined to be capable of being mounted to an actual firearm and functioning as a suppressor on an actual firearm, then you've committed a major violation of federal law, and will end up serving time for it. The point of a suppressor on a BB gun is a bit moot, anyhow - there is no expanding gases to contain with a BB gun (this is what a suppressor for a firearm does), so the difference in sound will be negligible, at best.