It is done with loads of patience and a whole range of abrasive paper. I would imagine from your description that the stick-tion is being caused by the lumps being tight in their slots L-R and there will be lots of witness marks to show it. One of the main rules is to always rub from bottom to top (towards the barrel), not from back to front (in line with the barrel). This is terribly frustrating as the movement is very short but it stops you making the lumps oval in cross section and allowing them to twist in the action.
Sorry, probably an unclear description but hopefully you get the gist.