How to properly assign an existing device to a new user?

Hello all -

I've had issues with swapping phones(and same extension/direct dial) from users who have left the company, to new users. 

For example, I want a new user Jimmy, to be able to use Katie's old phone and phone number. Katie's user profile/device etc is all still in the database. I'd want to purge Katie from the database, and transfer the phone over (make a new user) for Jimmy.

Can someone walk me through the correct order of operations here? I've had issues where the old phone user name shows up, or the phone just wont work with the new extension.

Thanks.