This is a very simple sub-assembly step. No big deal at all.
Worth noting again that if you're going to build yours with electronics, you might do this step differently.
Here are the parts you need for this step. One disk, and one thick ring.
Put glue on the back of the thick ring, then press it into place on the disk. Try to align them as best as you can.
All done. Give the edges a light sanding if you want. A quarter of this ring will be impossible to sand once you get the whole thing assembled, so if you're going to freak out about seam lines, now is the time to clean up the ones on this assembly.