A bore guide. This is the one I use - https://www.possumhollowproducts.com/BORE_GUIDES.html
- but there are others.
Some people buy long pipecleaner-like things to clean the gas tube, but I've never used one.
Clean the bore. Take the bolt out of the bolt carrier, take the extractor off, and soak the bolt in cleaning solvent. I use Ed's Red, it is cheap and effective, but most people aren't that cheap. Wipe dry and reassemble.
I use Lubriplate on the rings but most people are happy with their favorite gun oil.