Just screw it on. If a flash hider has a more solid part, put that part toward the bottom. Some people like to ensure any flash will not be directed straight up... so they index the FH so that there are no holes pointing directly up. To each their own.
I like crush washers, they are easy... just snug up, then tighten until you are happy.
And you DONT need an action block.... you just need a vise, and some wood blocks or leather, and chuck up on the front sight base.... and tighten away.
Dont forget to have it permantently attached by pinning and welding.... or complete welding.... recommended....