https://smashboards.com/threads/i-need-a-weapon-master-chief.452763/ Not too hard really(though I can't help as much as back when I was a Mod)
-First make sure it's either open for takeover, whether it means the original poster of it has been inactive for a severely long time(best to speak to a Mod to help work with in these situations), or they already gave you the go ahead to do so.
-Second is you must have a literal post exist before the first post in the topic.
https://smashboards.com/threads/i-need-a-weapon-master-chief.452763/ Here it is for convenience's sake. If you don't have a post exist before that, that means you'd need a new topic to properly be the owner of it. There used to be a tool to transfer over thread ownership, but we have a new Xenforo forum, and that's not currently a feasible/safe option to use programming-wise. Hopefully a new one is gotten that day.
-Finally, once you have the post selected, a Super Moderator, Admin, or a Moderator of the section your old post is in can move that post over. You'll have to update the post to reflect all the changes, like making it similar to the old opening post of the new topic, etc. My personal suggestion is to have that mapped out beforehand, then update either the post before or after it's moved. Then, once your post is updated, a Moderator can remove the old opening post from view, making you the new topic owner.