The easiest way for me is once the handle is done as such and the blade still safely wrapped in tape I put wet and dry on a flat surface such as the edge of the bench but with an inch or so overhanging the bench. I then put the spine firmly and squarely onto the wet and dry and with a single action draw the spine in a straight line towards me and over the edge, sometimes back and forth if a scratch is deep. Then I move the wet and dry towards me, just an inch so a fresh area is exposed and repeat the process, it really does not take long to get a great finish on the spine as you progress up the grits. You will surprised how long jus a sheet will last doing this as you only use a small area at a time. The same method works virtually all the way around the spine. Hopefully I have explained it correctly.
I suppose it is hand sanding in reverse as the paper is firm to the bench but the knife moving and as Tracey says you have to go to hand sanding for the best finish, something I learnt a while ago from advice on here. The other tip I was given was to use decent paper.....hence why the postie brought me 400 sheets of Rhynowet papers today, I wouldn't use any other papers now.