The head does not have to be removed... but take care not to drop a valve into the cylinder... otherwise you are screwed :P
All you have to remove is the valve cover and spark plugs.
Follow the firing order: 1-3-4-2
Turn each cylinder ALMOST to TDC on compression... this leaves a small space in the combustion chamber. Stuff a long length of cord/rope through the spark plug hole to fill that space and hold the valve in place... be sure you leave some so you can pull it back out
Turn the crank a little bit to make a snug fit... but not too tight.
Remove all the rocker arms... then the retainer and spring for the cylinder you are working on. Fairly self explanatory from there... each one is the same... replace the seal, put the spring and retainers back in... pull out the cord... crank it to the next cylinder.... stuff the cord in the chamber.... etc etc...