Practice practice practice, that's how i did mine, took about 3 month's, before it went all the way back without hurting.
in the bath, or in the shower, did it as much as i could. time is the key.
As other guys have already said, try to stretch it carefully overtime, but if the skin is not flexible enough, getting dry and with tiny cuts, you'll need the knife, not necessarily to remove all the foreskin but the narrow part at the tip.