If you mean "Synchro Summon"...
- Ensure that you have a monster with "Tuner" beside its Type, an appropriate Synchro monster to summon, and non-Tuner monsters all on your side of the field.
- Your monsters' levels, including the Tuner, all must total to the Synchro monster's level, no more no less.
- Send all the monsters you're using for the Synchro Summon to the Graveyard. Tokens can be used too. Declare the Synchro Summon.
- Place the Synchro monster that you're summoning onto the field.
If you're talking about making a monster a Tuner, there are several cards that can temporarily turn monsters you control into Tuners. Read the card texts carefully.