Depending on your number of paladin and warlock levels, it is pretty easy. Click on the cog icon on the character sheet. In the bottom-left-hand corner, type in the number of warlock spell slots in the correct fields of Spell Slot Modifiers (including any mystic arcanum). If you happen to have paladin and warlock spell slots at the same spell slot level, just remember that you are spending your warlock spell slots first (unless there is a reason for you to save warlock spell slots). That way, they recharge on a short rest.
