Get an old SunTour freewheel and rebuild it. As simple as they come. No special tricks involved. Unscrew the face plate and figure it out. Also a freewheel that's worth doing. If you get that far you should feel confident about doing most others. I would not bother with any current production freewheel. Do not attempt Shimano. Basic procedure is the bearings won't work if the races are full. Little 1/8" bearings. At the factory they will leave out 6 or 8 or 10 bearings. You can pretty quickly figure out if the freewheel in your hand works best two or three or four short of a full race. Repack with Phil grease or any clear synthetic grease. No, grease will not make pawls stick. Will stick if you ride really cold, I used to ride at zero Fahrenheit and that was too cold for grease. The other problem frequently encountered is the pawl springs are out of place or bent. Bend them back. If broken find a similar freewheel and cannibalize. Of course clean out rust and gunk. Once greased a freewheel body will run basically forever. All this was worth doing back when it was simple to buy replacement cogs. Now the primary way to get cogs is to cannibalize.