Any customizer will tell you that the best method to begin taking apart your figure is the "boil-and-pop" method, described below. This allows you to remove the head and limbs from any Star Wars figure with the stems intact and no damage to any parts of the figure. This is useful whether you want to do a "head swap" or want to apply a BASE COAT of paint.
Steps 3 and 4 describe splitting torsos open. Step 6 is an alternate method to split a torso that doesn't require a knife.
Place your figure in a simmering pan of hot water for several minutes. This makes arms, legs and heads very flexible.
Being careful not to scald yourself, remove the figure from the hot water, then quickly tug the limb from the socket. The limb may stretch as if it's going to tear, but this is unlikely as vinyl is very resilient at this temperature. Keep pulling firmly and the limb should eventually pop out. The stretched stem will return to its original shape. You've just completed the boil-and pop.
Now, if you need to take the figure apart further you can split the torso section. Insert a thin scalpel blade into the neck hole and cut the post that holds the front and the back together inside the torso.
Use a scalpel to cut the glue along the seams of the torso.
Carefully pull the torso open.
Another recommended method is to use needle-nosed pliers instead of scalpels. While the figure is still hot, insert the needle-nosed pliers into the neck hole until the tip reaches the post that connects the front and rear halves of the torso and pry the halves apart. For some figures (Han Carbonite, TIE Pilots) this is extremely easy. For other figures (Luke Dagobah) this can prove to be quite a difficult task.