You can set the buffs in your possession to the empty buff slots in your equipment items.
You can also acquire more buffs by removing them from other items using the "Remove Buffs" option.
Items with buffs can also be found in dungeons or bought in the Shop.
If an item has no free buff slots, you must create an empty slot using either "Remove Buffs" or "Erase Buffs" before you can set any more buffs to that item.