what do I need and how do I clean the backed up oil out of the pistons?

TO CLEAN OIL AND CARBON BUILD UP TRY AN INJECTION CLEANER. SEAFOAM WORKS WELL AND IS SOLD AT MOST PARTS HOUSES. IT WILL ONLY TO CLEAN ALITTLE BUT ITS A GOOD START

I think you mean oil is ponding in the spark plug tubes, if so this requires installition of valve cover gaskets with spark plug tube seals! Also a mechanic to do so. Pretty tough job for a DIY!! Or an experenced tech. for that matter.

I've always used water from a spray bottle to clean the combustion chamber if its a carbon issue in the cyl.