Selective breeding is what it sounds like, once your shrimp breed separate the ones that have a favourable feature (ie different colouring, in the case of the crystal reds, more solid and white colouring).
The only problem you might encounter is what you'll do with the shrimp you don't want to breed.
Don't just go and buy higher quality crystal reds if you've never had them before. It's a waste of money as most of them will die. If I were you, I'd try to buy a A and S grade mix and selectively breed them until they're all S grade (remember to introduce new crystal reds as the massive amounts of inbreeding will make the offspring genetically weaker) and then buy a few SS grade online and selectively breed until they're all SS grade.
Selectively breeding may be more practical in something smaller like a 10G because of it's size. It will be difficult to try and pick out certain shrimp in a 30G (with about 100+ other shrimp in there).
What experience do you have with shrimp?