Take a dowel and make a lengthwise slit in it. Cut a strip of 240-300 grit emery. Put one end of strip in slot and wrap the emery around the rod. You want it a slip but snug fit in sizer. Add oil to emery, a fair amount, and put it into the sizer. Roll the sizer up and down your thigh. Keep the emery strip moving in and out of the lower portion of the sizer. Lee sizers do all the work in the bottom inch or so.
Clean well after about 30 seconds. Size a few bullets and measure. Keep repeating until you get the size you want. As you get close it doesn't hurt to go to a finer grit but realize it will cut much slower.
For a nice finish add a little Flitz polish or JB bore cleaning paste to a snug patch and spin it in a drill. Run that into the sizer for 15 seconds and it will make it smooth as can be while removing essentially no material. I don't bother with this step, I find a 400 grit finish does fine.