When I cut a tube , I wrap a strip of paper around the tube and use it as a straight edge to draw aline around the tube. Remove paper, lay tube on the cutting board. Lay the flat part of the x-acto blade on the line and the tip just up and off the tube. Now PUSH the knife AWAY from you. You'll find that as you push the knife along (keep the tip up & off the tube) that it's like driving, but insted of driving a car your driving a knife. Your not trying to cut the tube at this point, your making a channel for the tip of the knife to follow when do start to cut the tube.
After you have gone around the tube a couple times lift the knife up so the point is in the channel and Pull the knife TO you. You'll find the knife follows the channel. Firm even pressure and several full rotations of the tube will produce a nice cut.
Use The flat of your finger nail, or the handel of your knife to flat'in out the bump the cut made on the inside of the tube ends.
I like to take a q-tip and dip it in thin ca, then put it on the ends of the tube. This makes all the paper fuzzies hard. Sand them off with some 320 grit sand paper. Now your coupler will slide right in. If it doesn't, sand more.