It's not overly difficult. The barrel on a mosin is simply torqued on as you can see in the video above. Then it's a matter of getting a barrel blank in the diameter of the caliber you plan on using. Then you have to thread the barrel to fit in your receiver, make a slot on the end of the barrel for the extractor if you're using a rimmed caliber, torque it on (if you made the extractor cut, then you have to align it properly). After the barrel is on, you must get a chamber reamer to make the chamber and after all that you have to do the headspacing, which is a whole other box of frogs.
I'm sure desporterizer will be around shortly as I probably missed a step or 2