You are too lazy for building trap or have issues building it (like me). You can also use diplodocus leveled in health. Set him on Attack your target and dind rex you want to tame. Shoot rex and your diplo will start attacking it. Since diplo doesnt do dmg only knockback he will act as a shield while you shoot rex with tranq arrows (darts recommended). As rexes torpor rises at around 80% he will start running. Just continue shooting until he drops down. For taming best you can use exceptional kibble, raw mutton, raw prime meat but raw meat will also work. After he is tamed you have a Rex!

Sorry for bad english and ty for reading