The easiest "no tooling" method is the "circular cut" method.
To do this, either use a tubing cutter to score a light line around the barrel, or wrap some tape around it, keeping it as even as possible.
Once you have a cut line established, use a fine-tooth hacksaw to make a shallow, one or two stroke cut on the line.
Rotate the barrel and make another shallow one or two stoke cut.
Continue rotating the barrel and making shallow cuts until you have a shallow line all the way around the barrel.
Then just continue making shallow cuts and rotating the barrel until you've cut all the way through.
This method prevents the cut from going off at an angle, and if you take care to get the original cut square with the bore, the finished cut will be very close to square.
Then you can use files to clean up the end and remove the sharp outer edge.
To re-crown the muzzle, chuck a large brass round-head screw in a drill, coat it with valve grinding compound and run the drill at medium speed as you move the drill handle in a circular direction.
All that's needed is to just break the sharp edge down to the bottom of the rifling grooves.
While this method works better on shotguns, it can be used with good results on rifles and pistols when you simply have no better tooling or a local gunsmith you can trust.