I suggest creating an alt character, each character is allowed a free loot bag.
The reason behind this, is simply that the gump is going to be confusing at first, and testing things out on a throwaway loot bag will help you in the long run.
Get familiar with adding and subtracting things from the gump, see how everything works. Once you have the basics down, then go back on your main, and add/subtract things to the that characters loot bag slowly over time.
[Helpful Hint 1]
Your going to probably want 3 Macro buttons. a Claim, a Grab, and a Claim -c button, for leather cutting/collecting.
UO has a built in Macro creator, found under: Paperdoll > options > Macro Options {last tab on the bottom left} which is great for simple macros like this.
Try using the same button for similar things; for instance:
(z) for [Claim (z+alt) for [Claim -c and (z+Ctrl or Shift) for [Grab.
[Helpful Hint 2 & 3]
As far as I know there is no return to defaults button for the Loot bag 'gump' options.
Lastly, some things are set as groups by default, deleting these groupings, means you have to then add each item, under that grouping, back to your list individually. This can suck.
I am not an expert when it comes to the loot bag, but I do know some things.
--Sturger--