Lesson 1 about Backstabbing. Drop the dagger. Why? I know what you're up too, I know how that weapon works and its limitation, and I'll fish you for a backstab and counter it.
Weapon for Training BS. Bare hand + Flip Ring. Why? You want to practice as many time as possible, not kill the fella and restart.
Where to start training? Remember the Firelink first Hollow Soldier you killed? Yeah, they probably be a good start for your roll bs. Kill all but leave the weakest and try to roll bs by the cliff. Teach you space management while your all flipping around.
Once your okay, get some move and confident going. Let say you get to Roll BS 2/5 attempts consistently.. then go to..
Dark Anor Londo for Iron Knight (kill the Balder girl). Why? He got some similar moveset like players does, and his slow.. so you can practice and get your timing right.. once you get to roll bs 2/5 attempts consistently..
Try Balder girl.. She move faster, some rapier moveset, and she parry.. so you got to get your timing better or get punish.. once you get the confident move on to PVP.
Your first week in PVP is hell.. you will have temptation to kill someone with a straight up fight, you will lose fight and get counter because your attempt to roll bs.. Don't and endure it.. learn exactly the mistake you done, time your movement more, learn all the weapon moveset, and how you miss to land the hit..
Once your pass that 1st week, you should be able to do RBS quite easily.. then start to learn the complexity of mind game, fishing, counter a counter bs, stuff like that..
and Have Fun!