take the spring off the shock, lift the front end in the air so the suspension hangs put top bolt in shock and extend it all the way, now compress is and put the bottom bolt on, take a sharpie marker and mark the shaft at the base of the shock body. pull the shock off and extend it measure from the mark you made to the shock body, add a 16th or so to the measurement and thats how far to shorten it. take shock and let the gas out of the air valve. turn shock upside down and put in a vise. dust cap on base of shock body should tap off fairly easily., pull it up and tape or ziptie it out of the way. take a small flat punch or socket extension and place it on the metal part of the seal head and tap the seal head into the shock untill you see the big metal circlip. CAREFULLY remove the circlip. pull up on shock shaft and it should now pull right out of the body. to remove the nut holding the piston on you may need to carefully dremel the top if it is peened to keep it from backing off. DO NOT get the valve shims out of order when you pull the piston off.
To make the spacer, go to the hardware store and get a piece of round stock delrin, drill a hole in the center then cut the spacer the correct length. (this is where the lathe comes in handy, you want to cut it perfectly flat. and machine/tool shop wll probably cut it for you for a buck or two) put spacer on shaft, reinstall piston with LOCKTITE RED on the nut, reassemble shock, then take it to the bike shop to get recharged.