If you have a stego already, a good strategy would be to ride your previously tamed stego with heavy plate equipped. You hit the stego with heavy plate to start, slowing it while aggroing so it stays in range, then switch to hardened plate (the slowing debuff should give you plenty of time to do so) then, equip a crossbow with tranq arrows (personal preference, but ultimately it’s up to you) the aggro on the stego will keep it close enough to be able to steadily headshot the stego, while you being in your stego will prevent it from retaliating and lowering its health. You will probably have to periodically hit the stego with heavy plate equipped to keep the slowing debuff active, but other than that you should be fine
Stegos have the new tlc which means in the wild they will have about a 50% damage resistance which will make it harder to knock out. With the info on knocking it out (how much it takes) multiply it by 2 to make it accurate to how much you need to knock it out! Hope this helped!
They are mildly slow, but you might want to get on a rock and shoot it with tranq arrows, takes 2-5 arrows and make sure you have bio toxin on you. Takes a while for them to tame, the best thing to beat that time is to get a sleeping bag ( Not a simple bed!) And sleep in it. When you wake up shove a hundred bio toxins down it's throat, wait that three more minutes, and you got a stego.