I'm a huge fan of custom 1/6 heads. The only problem is that you typically have to mount them permanently on a body. One of the fears I have is damaging the paint jobs if I want to change the figures clothing. Here's a method I devised using Power Team bodies.

First: I mount the heads upside down using poster tac.

Second: Using Alumilite, I fill the holes in the neck the caster provided. Be VERY careful not to spill the Alumilite. After the Alumilite hardens, I sand the neck bottom smooth.

Third: Using the PowerTeam (PT) head as a guide, I mark the PT neck lining the heads up by the ear position.

Fourth: I tape the PT head with tape, in order to mark where to cut.

Fifth: I cut the bottom of the neck off.

Sixth: Plugging the hole with electrical tape, I then fill the neck bottom with Alumilite.

Seventh: After the Alumilite hardens, I sand the top smooth.

Eighth: I superglue the head to the neck bottom. Using "green stuff" I fill any gaps and match up neck contours.

Ninth: After the putty dries, I sand this smooth.

Tenth: I prime the head black in order to paint.

Final: I paint the head. Here's a link to a great site on how to paint heads.

One thing to remember, PowerTeam heads have two different connectors. One is plastic, it'll be the same flesh color as the body. The other will be a white vinyl. If you are using the non-vinyl head connector, you will need to cut notches in the bottom part of the neck in order to make it easter to remove without breaking the neck peg. The vinyl connector is much tougher and will not break as easily as the plastic connector.