I've done it a couple of times, takes a while usually. You just concentrate and use your pelvic muscles to contract and it feels pretty good. Trick is to keep a steady rhythm.
For the first time you might try wanking until your just about to cum then stopping and finishing off handsfree...
Nothing at all to be concerned with.. I was apprehensive the first few times, but the experimentation phase is wwonderful..... research it on the net.... and watch some of the estim vids on xtube..... I guarantee you'll enjoy it. I've introduced a few guys to it and, to a man, they came HARD ( not to mention hands free ) the first time they tried it.