When you're ready to install the forward assist on an AR-15, it's important to use a good receiver holding fixture to prevent damage from the vise. The forward assist assembly is made of only three parts: the forward assist, a return spring, and a roll pin.
Be sure to oil the roll pin and grease the forward assist and spring.
Start by securing the receiver in a holding fixture and clamp the fixture in a vise.
Slip the return spring onto the forward assist...
...and insert the assembly carefully into the hole. Push in and rotate as necessary so you can see through the hole for the securing pin.
Use a 3/32 punch. Insert it from the bottom as a slave pin. Start the roll pin into position with a roll pin starter punch.
Then use a standard roll pin punch to drive it the rest of the way home. This will drive out the slave pin.
The forward assist assembly should move freely back and forth.