Mark along the top of both walls where the trusses will sit. Typically mark at either 16 or 24 inches on center.
Place either end of the truss on the marks on the top plate with the help of an aide or assistant. Ensure that you both use the same mark or your truss will be crooked.
Grip a 16D framing nail at 45 degrees approximately two inches up from the bottom of the truss and hammer it until it sinks into the wall's top plate enough to stay on its own. Grip the bottom of the truss and drive the nail in until the head is completely buried.
Nail the other end of the truss in the same manner.
Check all nails are sunk down into the top plate of the wall. Verify that no nail pokes out the other side of the truss. If it is sticking out, the nail doesn't have a strong hold. Remove any such nail and nail again into the wall's top plate.