Ignore the large nut. There are nine countersunk headed screws which hold the ring in the middle of the outer plate. These have to be undone and they can be a pain if their slots are worn or otherwise mutilated. I suggest that you start with the middle one of each or the three sets and undo those about one full turn. Then undo the outer pari of each of the three sets about three turns each. Then go back to the centre ones and undo those about another two turns. Then completely undo the outer two of each set and finally fully undo the centre ones and then you can remove the ring, the outer plare of the servo clutch, the friction disc and so on. Refer to your books or parts list as to what goes where and if any doubt lay the parts out in order on a clean sheet of cloth or paper. Eventually you will get to the stage where the clutch shoes are visible. Now you have to remove the spring with acts as a retaner for the slotted nut which holds on the clutch shoe carrier. Remove that nut by putting the bike in gear, blocking the rear wheel by using a piece of woos across the rear frame and under one of the spoke nuts to stop the rear wheel rotating and then undo the slotted but. It will depend upon what model and age of bike you have as to what you will find under that nut. After all that you should be able to pull out the shoe carrier and shoes. If it has been put on with sealing compound on the splines you might struggle but it will come off eventually and then you can see the inside of the clutch drum. Follow all the steps in the books and ask questions before forcing anything.