You need only tranq arrows, a crossbow, a flyer, narcotics, and a type of meat for this method

Look for a Rex on a mountain and get on a level above it. High enough that it can’t get you but low enough that you can see it and shoot it. Then shoot it with tranq arrows until it passes out. Fill its inventory with narcotics and the meats of your choice, feeding narcotics if needed.

And there you go. Your new Rex is tamed

Hope this helped!!