The coke is removed by "mechanical means" i.e. whack it out with a cold chisel or whatever comes to hand, without cracking the fitting. A pneumatic needler would be ideal if you have it, most of us don't. A Dremel tool might help get rid of the hard parts without risking a blow to the manifold.
The most important tool for replacing a head gasket is a torque wrench. If you don't own one, I'd suggest don't buy the cheapest, and look for one that has a "snap" or "pop" setting for torque, not the old "analog read the scale" kind. Digital torque wrenchs are likewise a waste of money, IMO. Treat it like glass--all you need to do is drop it once to knock it out of calibration.
Then check the engine shop manual, buy one if you need to. The head must be bolted down in a specific sequence to a specific torque setting, and then re-torqued typically a month afterwards. The bolts themselves MAY be re-useable but the manual may tell you to replace them. Sometimes they are designed to stretch during installation--`and only be used once.
The rest is pretty much just simple wrenching. Although while the head is off, that's the perfect time to eyeball the cylinders and rings, send out the injectors for a rebuild or servicing, attend to other things that never get done while the engine is expected to be all in one piece and working.
A "dry" compression test is called ust hook it up and test. For a wet test, you add some oil to each cylinder first, in order to stop any blowby past the rings. That tells you if the rings need replacement.
Of course if you are pulling the head, you can also do a leakdown test to check the rings. You pour some fuel into each cylinder, to the top, and then go away for a while. If the fuel is gone, it has leaked down past the rings and it is ring-piston-cylinder time.
Some of this stuff you would find described in any shop manual for almost any common diesel engine, your local library is guaranteed to have something that will give you an idea of what's involved, if you can't get hold of a shop manual for yours easily. (Some are on the web.)