Hi and welcome!
Basing is really fun and a nice part of the miniature-hobby. The way you do it is up to you. Sometimes I paint both base and mini first before assembling, sometimes I put the mini on the base before painting anything. A few things I always consider:
- do I need to drybrush the base? rocks are easy to drybrush, but if your mini is already attached to it and also painted, you can mess up the nice paintwork on the mini. If you want to place the mini on the rock before painting anything, paint the rock first and then the mini (be careful when painting the feet

)
- can the mini be easily attached to the base? If this is the case, and no major putty work is required for a stable connection, I paint mini and base separate. No chance of messing up, only a bit of paint can dissolve when you glue the mini on the base.
The way you attach your models to the base is also up to you. If you don't want to use the tab (which is the most secure way to glue it on a simple base-with-slot I believe), one or more pins through the feet are quite secure. My experience with cork is that it gives enough support. But that depends on the thickness of the cork I guess, you can also drill a hole through the cork AND the base, and glue a long pin through the whole thing. The only problem I have had: very small feet, making deep drilling impossible.
Anyway, have fun and show your minis!