Get a good bow/crossbow/longneck

For ammo use tranq arrows/tranq darts/shocking tranq darts

Build a 3x3 stone foundation, place doorframes on the sides, build it 2 on each foundation and place ramps on any side of the trap

Get the theri's aggro and go up the ramp and fall through the doorframes

If you did it right it should be in the trap

Now knock it out with your tranq of choice

Make sure to bring lots of narcotics because it loses torpor fast

More Therizinosaurus Taming & KO Tips