Take the left hub off and the 2 axle nuts then slide the rear sprocket and sprocket hub off athen take the castle nut from the left hub and flip it over and screw it on till the axles threads are flush swith the end of the nut and hold the bord up to the axle and then start whaleing on it
Something you might think about is if those axle bearings have been there for some time, you might change them while you have it out, if not clean it up to make sure you can get it back apart if you have to change them.