Step 1: make 4 (or more) stone gates and a large bear trap

Step 2: put 3 gates down in a box like formation then bear trap in the middle like this |_| (| means gate)

Step 3: get the Rex on the bear trap and place the last gate behind the Rex trapping it

Step 4: using tranq arrows, toxicant arrows, tranq darts, shocking tranq darts, or creatures that deal a lot of torpor knock it out

Step 4: just go kill a prime dinosaur for prime meat then put the prime and regular meat in the Rex’s inventory

Step 5: once it tames have fun with your new Rex like I did!