I practice regularly. I could say a lot on it but Ill try be brief.
Don't see it as penance anon. The whole goal of it should be working toward not sinning, and therefore not having to use the whip. You should be doing an examination of conscience every night. During this, pick a particular nasty habitual sin and assign yourself a number of lashes (5-15 is pretty decent imo, depending on how badly you failed). Do them while meditating on those particular sins, you could also pray eg. I do "Sacred heart of Jesus, have mercy on me" and whip at "mercy". Do not go overboard with it. The next day focus on that same catagory of sin. Make sure you pick one sin and remove it through this method, then move onto the next.
It's all up to you though. If it doesnt improve anything over a week, maybe stop or try altering a bit. Personally with this method Ive been able to stop a whole number of sins. I managed to stop pretty much all pornography within 4 days.
As for the type of whip, I just use a bit of rope Ive tied a knot in the end. It just needs to be enough to sting, but you definitely dont want to do any damage to your body.