Step 1. Have Instakill Perk and Celestial Reading Perk as these two perks will greatly help you in the grinding part of this process
Step 2. Once you've activated Celestial Reading make your way into mementos and travel to a low level floor
Step 3. Once on the floor kill low level enemies until fusion alarm is triggered.
Step 4. Make your way back into the velvet room and fuse the two random personas you got off enemies
Step 5. Once the fusion is complete take the desired persona you wish to max its stats out and strengthen it using the persona you just fused.
Step 6. Repeat until Stats are 99 MAX
(And don't forget to Register your progress)