Patchouli sorta explains it to you. In Japanese.
The first slot obviously is for your doll which you want to reincarnate.
It resets basically everything. I think the only thing that doesn't get reset is the 'Encountered in place, at time'
The second and third slots(Inheritance) are for when you want to pass on IVs from other dolls.
If you set one doll, two of the reincarnated doll's IVs will be based on this doll.
If you set two dolls, three of the reincarnated doll's IVs will be based on either one of the dolls, separately.
The fourth slot is for when you want to give a particular 印 to the reincarnated doll, which boosts a particular stat by 10%.
(印 has a lot of meanings, and I don't feel like finding a good translation for this, for now. 'Mark' might probably work, if it itself didn't have a lot of meanings)
The fifth slot is for when you want to either
fix all IVs from the reincarnating doll, or
pass on a particular IV from the reincarnating doll, or
when you want to inherit more IVs from the inheriting doll(s).
For the fifth slot, except for the first option, you'll have to get it from somewhere else.
Thankfully, in this game Skill Cards (TMs in pokemon) are not one-time usables, so try using it.