One problem you may have is not docking it to the proper length...too short or too long. Or, you may not have it clamped enough to completely stop blood flow...which would be really bad.
I've never done it myself and unfortunately, did not get to watch when our vet performed it on ours, although I'd watched her do other litters. I am sure I could do it myself without a problem, but I would just rather our vet be able to see them while they are young, so it just makes sense to have her do them, at least until a point in time when I am completely comfortable with it. After only seeing it done once, that was not something I was comfortable trying on my own.