This is known as a "piston soak." It's been discussed here many times, using any number of a variety of popular cleaners/solvents.
The things you generally want to keep in mind are: Don't put more than a couple of ounces in each cylinder so as to avoid hydrolocking the engine upon trying to start it up again; for optimum results, the pistons should all be as close to the middle of the cylinder as possible -- why? Because otherwise one or more will be near the bottom of the bore and the fluid you put in those pistons may just pour right past the rings; after letting whatever you use sit in the cylinders for a few hours, turn the crankshaft by hand to move the cylinders a bit to let the cleaner get into the ring grooves; let the stuff sit in the cylinders overnight so that it has a chance to work; plan on replacing oil/filter immediately after the soak; start the car again at night if you have neighbors who dislike thick clouds of choking white smoke, since this will be issuing from your tailpipe for at least a dozen miles upon restarting.
Finally, if all you're trying to do is clean the piston tops, vs. free stuck rings, I suggest using Lube Control and Fuel Power on a regular basis instead.