Be very careful! If you do it in the wrong order you can snap the damper rods which cost about £20 each!
You can do this with the fork in one piece, or dismantle it and do it one leg at a time by undoing the crown and brace.
First thing to do is to reduce the pre-load on the springs. Turn the red adjusters towards the "-" as far as you can then remove them using a 2mm allen key. Underneath them is a circlip. Remove that with a small screwdriver while holding your hand over it to stop it pinging off somewhere unreachable. Now you can undo the top cap on one leg to remove the spring. Drain the oil out of the leg then you can undo the foot nut (at the bottom of the leg funnily enough). It should be a 15mm socket but I have known some to be a different size.
Now you should be able to slide the legs apart with very little resistance.
Next get your slider and use a screwdriver to remove the first seal. You'll have to go a bit at a time rotating the leg and levering the seal as you go. I guarantee that you'll probably damage the seal even if you're careful, hence the need to replace the seals (I always replace the seals as part of a service anyway). Underneath the dust seal you'll see a huge spring clip that holds in the next seal. Remove this being careful not to score the inside of the leg. Once this is out you can then remove the oil seal in the same way as the first one (this seal is a lot tighter than the other - you may want to use a rag to protect the finish on the leg). Once this seal is out you'll find a large washer underneath which should just fall out at this stage. When that's out of the way the bushing should come out without too much effort (sometimes you may need to use a small screwdriver to lever up the edge).
Now you just need to slip the new bushing in and reverse the above steps.
When you fit the new seals you'll have to find something to push the oil seal in straight otherwise you'll damage it. Marzocchi do both a seal removal tool and a fitting tool, which aren't too expensive, but you'll probably only use them once or twice. The outer dust seal can be pressed in by hand but it's best to remove the "spring" on the outside before you do as this is easily damaged. When the seal is in then you can put it back.
When you re-fit the stanchion be careful not to damage the seals. If you smear some of the oil around the edges of the seals it generally makes fitting a bit easier. Also if you turn the leg at the same time it can help.
The oil you need for re-filling the forks is 7.5 weight. If you can't get any then you can make some by mixing equal volumes of 10 and 2.5 weight.
I'll have to check how much you need because all the forks have different amounts (they can change year on year even for the same model).
If you let me know the exact make and year then I can let you know (the model year is stamped on the inside of one of the dropouts in a circle arrangement).
