About to replace my cassette. How do I attach these two loose cogs onto the rest?

You need a freehub 😀 and then just slideeeeeeeeeeeeee….

They set on there, aligning with notches, and the lockring holds it all in place. As a tip, there’s a small dot marking on them which helps you get them aligned with more than just feel. If you don’t have them aligned right the lockring won’t tighten down right, they’ll sit crooked, and you’ll have all sorts of problems.

And be sure you torque things to spec.