Drop pan, move it out of the way as much as you can. Take out the two bolts that hold on the main seal cap, mine were 12 point heads. After that the main seal pulls off. To get the stock rope seal out of there, thread a small screw into the rope material(being careful not to score the crank or seal groove). Then pull on the seal while rotating the crank in that direction and it should come right out.
As for doing a new one so it doesn't leak, I don't know. The instructions for the neoprene style seal are to clean the groove around the crank, then oil the lip of the seal that faces the crank(keep the other side dry) and slide it in. The side seals are made of a material that expands rapidly once you give them a quick dunk in mineral spirits. You dunk them, then quickly bolt them together. You put a small dab of rtv on the seal cap to cover the area between the side seals and the rounded crank seal.
There are other styles of main seal but I don't know what works best.