Didn't say the game doesn't have glitches. Said equipping potions isn't glitched.
Those various "belts" allow a character to carry more of a given type (potions/tonics/grenades) than would have been possible without it. You still need the proper ingredients, and a slot for the flask.
If you're asking how to Create potions/tonics/grenades:
1. Find the recipe.
2. Find the ingredients listed in the recipe.
3. Proceed to "Equip" below.
If you're asking how to Equip potions in general:
The first (left-most) slot is for healing potions only, (and the quantity showing is a shared by the entire Party.)
The other slot(s) can be equipped individually by each character (with limitations; e.g. a non-mage can't equip lyrium.)
1. Select the character you want to equip with more (or different) potions.
2. Go to an "Equip Potions" supply bench.
3. Click and HOLD on the type you want from the available ones on the left.
4. DRAG it to the second (or third, if you have it) potion slot (if something's already there, the selection will replace it.)
5. Double-click on the slot until you've maxed-out the number it can hold, (or don't have enough of the necessary ingredients to create another of that type.)
If you are asking how you Unlock the third potion slot:
It's the perk called "Tempered Glass Flasks", and it's found in the "Forces" Perk Tree.