In addition to the extensive instructions of eddymercx3 and deepcover two more hints: - once you've inserted the valve and half inflated tube push the outer bead (the one that's already on the rim) to the middle of the rim, where it is deepest. In that way the other tire bead get's just that bit more space - when you worked up the tire bead most of the way, let out the last of the air that inside the tube. To me it's always a surprise how much of a difference that makes. (and of course the temperature of the tire, which really makes a difference as well)