Its easy to tame a rex or any dinos that are not fast runners

heres some tip

1.tame a tapejara put a saddle then shoot the rex while you and the tapejara is flying

2.shoot it with tranq darts/arrow or toxicant arrow

3.if its torpor running land then catch it or just change seat in front or back but you must make that your tapejara is following you or else it will land.. but dont worry tapejaras wont lose stamina if you change seats it will just regain some stamin

4.if its knockedout put some walls so nothing attacks it or just kill some predators that are near in the rex or it might disturb you from taming it

5.if tamed put the saddle then have fun :)

6.if you accidentaly hit it you will lose some taming effectiveness and a bonus level