Honestly, these are really easy to tame.
Get 100 tranqs, 60-80 narcs, (i reccomend 80 just to be safe) And 100+ fish meat.
Get 1 stone base, 2 tall stone walls (or metal), and 6 stone ceilings (To have space to land a flyer).
Make sure the walls are on only 1 side, so it should be 8 single walls tall. Or you can use a Tapejara.
Pick a flyer with a decent amount of health, just incase something goes wrong. I recommend a high-level Argie.
Build it by a Rex, Land and shoot. Chase it when it starts running away.
After its knocked out, you can put you flyer on Neutral, so it can distract anything.
Rexes eat fish meat quicker, so use that. Monitor the Rex's Torpor levels, and feed it narcs.
Now you have a Rex! Now you can get another one so they can breed, and now you have a Rex army.