I can tell you the biggest things: 1 make sure you get all the water out of the tank and the bowl. I used a vacuum in this video. But you can use some old rags if you like.
Now on to the seal or johnny ring. That’s the wax ring. It’s very important that you remove all of the wax and any flanges that were part of the wax ring. Always replace the bolts with new ones.
Replacing the bolts will just make everything so much easier for you. For one, most of them would’ve been cut shorter once they were installed and it’s the length you want when you are trying to line the base of the new toilet up with them. Now chances are they are now going to be too long and will need to be cut down in order for the dust caps to fit on them. And that’s OK. Just use a hacksaw to cut them.
It’s also always a good idea to replace the supply line at the same time. For a few bucks, it’s not worth not replacing it.