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