Saad,
Are you in the U.S.? If so, I'd recommend that you only let someone qualified to sleeve a barrel do the work. That is, someone that has past experience and references.
Sleeving involves cutting the barrel off at a point just ahead of the chamber. Usually around 3" or so from the breach. The remaining chamber is bored oversize to allow for a predetermined wall thickness of the new barrel to have after fitting in the oversize chamber. The face of the remaining chamber must be machined perfectly square with the bored out chamber. Then a new barrel is turned to fit in the old bored out chamber. The new barrel should be restruck (filing and polishing of the barrel to control its profile). Then the new barrel is soldered, brazed, or welded in place. The traditional method is soldering the new barrel into the old chamber. Then, additional filing, blending of the profile of the assembly is done along with polishing and solder reassembly of the spacers to regulate the point of impact, assembly of the ribs and forend lug, cleanup of the soldering, more polishing and reblueing.
If it sounds like a lot of skilled work, you're right. If it doesn't sound like it requires much skill, I didn't explain it well enough to do it justice.
