Client 12:

On Client 12.50 you do not need to follow this tutorial. Just use client-side features by clicking on "Manage Loot Containers" on any Backpack.

Possible Modes:
Skipped Loot - Loot all except items added to the list
Accepted Loot - Loot only items added to the list

Open menu of random item and click "Add to Loot list"

Client 10:

Quick loot commands and how to use them

!quickloot list - list your current quicklooting items
!quickloot monsterName - select loot items based on specific monster loot (eg. !quickloot dragon)
!quickloot add,itemName - add item to quickloot by name
!quickloot remove,itemName - remove item from quickloot by name

Write !quickloot in-game to display all commands for this system.

Choose your command write, for example: !quickloot monsterName.

It will display modal window with items that can be looted from specific monster.
Choose your loot what would you like to get instantly after clicking a monster corpse in your backpack.

To check out what you already have in your list use !quickloot list command.
It will display you modal window where you can remove added loot item by simply clicking remove.

Final phase simply kill the monster you've choosen and try to open the corpse.