tjonh2001 , you can build your own silencer AFTER you have filled out and paid for the form 4 from the ATF , the form 4 is a one time construction license that allows you to build and register one silencer per form. The one silencer that you make is your and yours alone you can't sell it , I have been told its possible to transfer them but haven't looked into it.
Building one can range from very easy to very hard depending on your style and what it is your wanting it to do. A simple and very effective silencer is a simple chamber and baffle setup but it will need to be a little bigger than the new K baffel desgined silencers as the K baffels are about the most effecient desgine being used right now.. If you have access to a lathe even a small one , making a silencer is easy. You can also have a machineshop build one but you have to be their with your paperwork while its being done and you have to take ALL the parts with you when you leave , the cost for having a shop build it will likely be alot higher than buying one outright.