How do I put my account on someone else's old ipad

One of the best ways to prepare a previously owned iPad is to follow this procedure:

What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support

This will remove all data - and remove the Activation Lock that will otherwise inhibit reuse of the iPad.

First you need to log out of the old account.

Settings -> Apple ID -> Find My -> Log Out of Find My.

Then go back to Apple ID, at the bottom you can log out. Then you can sign in with your own Apple ID. I would recommend doing a reset after logging out of the old Apple ID, so as not to have the previous owner's stuff lying around, however, it's up to you.