For full Forest Titan taming details, get the taming calculator app.
To tame a Forest Titan, you must destroy the corruption on its body. Taming effectiveness will drop if you hurt the Titan, so try to focus on the corruption. Titans come pre-equipped with a Platform Saddle, however, they are piloted from a seperate cockpit. Titans are a 24 hour temporary tame.
Pro tip: Don't use explosive weaponry and really bad rexes with no saddles like the devs did. Fab Snipers for taming it with little/no collateral damage, actually decent rexes + imprint + saddles to kill it
Try to hit the coropted glowing things around his body. Ice you have destroyed all, it should fall down on the ground knocked out. You do not need any taming food for it, Just go up to it and click tame to tame it. No need to build a saddle because it already comes with one.