Here is pics of my lapping tools for .12, .21 and .15 engines.
It made out of garolite LE, clamp type.
Procedure as follow-you apply small ammount of laping compaund inside the tool, then stick piston inside the tool and tighten the side screww untill if you turn piston you will feel that metall is laping. Next step is turning piston and mooving it forward and backward ( 1 turn-2mm move). After about 3-4 turns you need to check piston for fitting in sleeve. I know your next question-how far to go-well here is personal preferences, like for Novarossi .12 I stop when piston stack from the top of sleeve 2.3-2.5 mm, but this is in my piston/sleeve sets, for novarossi original I really doen't know-neber deal with them.
After you finish lapping, make sure you wash piston and sleeve really good-laping compaund stays there badly and it really need full attention. If some of compaund left-you set will gone in seconds after you started. I use strong soap in hot water-boiled and then aceton or laquier thiner.
If you have more questions, please feel free to ask