This is a tricky job - there is a special tool that is needed to lift
the upper half of the head off.  If it's not lifted straight up, then
the shoulder at one end will crack off.  This same tool is used to push
the upper half of the head back down, but at least this is possible with
careful tightening of the bolts.  Also, there is no gasket between the
upper and lower halves, so a special sealant is applied with a roller,
similar to a paint roller.  If you put too much on, oil passages are
blocked.  If you use too little, then you get oil leaks.

This job is best left to someone who's experienced.