Removing a Head Gasket can go from difficult to really hard depending on what kind of engine you have, what the Cylinder Head is made of and what caused the Head Gasket to fail. All engines use some form of camshaft to make valves open and close in the cylinder head. These valves allow Fuel & Air in and Exhaust Gas out.
A push-rod engine has the camshaft below the cylinder head. These are the simplest to remove the Head Gasket on. It is just a matter of removing the head by loosening the bolts slowly and evenly from the outside in. This means you loosen each cylinder head bolt a little bit at a time working from the outside to the middle and then repeating the process until they are all loose. The workshop manual for your car should show the correct loosening process
An Overhead Camshaft engine has the Camshaft on top of the cylinder heads. These are more difficult cylinder heads to remove as you have to disconnect the Camshaft from the Timing Chain or Timing Belt which turns it. This also means that when you go to put it all back together you have to making sure the Camshaft is lined up correctly with the engine timing. Failure to do this can destroy the engine as Valves can collide with Pistons. Likewise you will also need to loosen the bolts slowly and in the correct order.
Once the Cylinder Head is off you should have a Machine Shop check it out to make sure it is flat and has no cracks. For it may not have been the Gasket that failed as the Cylinder Head may have had some cracks in it which let gas or coolant escape. The shop can check the Cylinder Head out and advise you as to what they can do with it. If it is a little warped then they may be able to skim it (take a slither of metal off it) to flatten it out or you could use a thicker Head Gasket to do the same.
Replacing the Head Gasket is the opposite of removal. Never Ever FOLD the Head Gasket as that will destroy it. You place the new head gasket on the cylinder block - some people may use some Head Gasket sealant at the same time to make sure of a superb seal (but be careful as that can affect the whole process by making the Head Gasket thicker). Then you replace the Cylinder Head and tighten up slowly from the center out as per the instructions in your Shop Manual and tighten to the Torque specified. Torque is the mount of effort it takes to tighten the bolt and a Torque wrench can tell you that. Bear in mind that a Torque measurement is against a dry, clean and non oiled bolt. Using oil can cause you to seriously over torque the bolts and could cause a cylinder head to warp.