Start at the top. YouTube 'wrist roller'. Make one and use it (like for $5.00). Then forearm curls (forward and reverse). Finally NO FKIN STRAPS.
Reps and sets as you do for other bodyparts. Right now I'm on a 20 rep schedule so yesterday I did 4 x 20 reps (increasing the weight on each set) for forward wrist curls (works the underside / flexors).
I must have jacked off enough in my teens to get some good forearms, lol. I never train forearms directly. I think just heavy deadlifts are a good exercise that hit them well. Maybe some reverse grip curls too.