1. Check PA law to see if any extra steps are required to own one in PA. (Here in Oregon, you just need to follow Federal law, and you're fine with no extra steps from the state, but this varies from state to state.)
2. Go to the person/dealer you want to buy the silencer from, they'll have the NFA paperwork you need to fill out. You do not need a "license" to own an NFA device, you must pay a $200 tax, and go through a 4-6 month background check. You will pay for the item and the NFA processing fee/tax up front, and wait for the paperwork to go through. Once it goes through, you can pick up your item.