I just did this the other day and it works very easily.... Use whatever strop dressing yer going to use (I used neets foot) Take a very fine cheese grater and shave some of the bar into a small glass bowl filled with a small amount of the dressing, mix with finger and apply as Josh showed above works perfectly, and very easy... I stored the left over in a small AS sample bottle for future use, should be about 20 years before I need to do it again :)
PS: I said glass bowl for a good reason, some idiot I know well used the wife's Tupperware and left green Cr impregnated into the plastic!!!!!