The steps of a piston are obvious but intimidating.
1) Find a tube that is nearly the size need to fit closely in the casing.
2) Make a top of the that fits on the top of the tube, and is thick enough to hold both the valve stem and also the valve guide hole.
3) Make a bottom for the piston with a piece of tubing that fits inside the piston tube. Solder a thin plate of metal to this piece of tubing. Drill a hole in the center.
4) Locate where the holes need to go through the piston tube, using some marking powder to define where the holes go. Cut out the holes.
5) roll copper foil, fit it to the size of the hole in the valves, anneal it, solder the seam with tin (higher melting point than soft solder) to make a tube. Insert the foil tube so that it extends a bit out of the piston, shape and smooth it, soft solder the foil to the piston tube.
