The best product to use as a piston soak to help free stuck compression rings is Berryman B-12 Chemtool Fuel Injector Cleaner (part #s 0116 and 0121).  Once you’ve removed the spark plugs, be sure to add enough cleaner to cover the entire piston crown with cleaner.  With inline motors with vertical strokes (like most 4 cylinders and straight 6s), this may only be 2 fl oz of B-12 Chemtool per cylinder, but with V-configured motors (like V6s or V8s), you might need a whole can per cylinder due to the angle of the pistons and diameter of the bore.  If you have a V motor and don’t mind doing this in halves, you can jack up one side of the vehicle in order to decrease the angle of the pistons.  This might cut the amount of product down quite a bit over the course of the whole job.   To perform the soak, remove the spark plugs, add to the COLD combustion chambers enough B-12 Chemtool to cover the entire piston crown, and put the spark plugs back in a couple threads in order to prevent evaporation of the solvents.  Regarding duration, we recommend soaking the pistons for at least 1-2 hours apiece.  The process will benefit from a little agitation, too, so if you can safely rock the vehicle from side to side occasionally, that will improve your results.  Obviously, you’ll want to be careful doing this if the car is on a jack or jack stands.   Once you’ve decided to end the soak, the cylinder will need to be evacuated of the cleaner/carbon mixture.  You can do this any number of ways including with a vacuum pump or turkey baster.  Note that B-12 Chemtool will damage most automotive paint and clear coats, though, so be cautious around such finishes.