Assuming you enter the fortress with item sliding, you can't open the juju bag before meeting LeChuck or the game will crash. Luckily, there's an easy solution for that: don't open it :). Instead, this is what you can do:

1. Before entering the back tunnel, scroll the inventory so both the crazy straw and the juju bag are visible, and position the cursor on the crazy straw so it becomes the current item.
2. Solve the bone puzzle, open the big door, and pick up the voodoo key.
3. While LeChuck is talking, hold Backspace and blindly move the cursor as close to the pan as you can.
4. After the cutscenes, open inventory, press Enter to select the crazy straw, and then press the right arrow key and Enter to select the green drink.
5. Escape the cutscene, spit on the pan, and spit on the shield twice.
6. Hold period to skip the dialogue. Don't move the mouse at this point.
7. Open the inventory; the cursor will be right on top of the juju bag! Click on the bag (or press Enter) and then quickly press Backspace, open the inventory again, press the right arrow key and press Enter to use the matches.
8. Hold period to skip the dialogue and then Backspace to skip the cutscenes.

One thing to note: after solving the bone puzzle, if you enter the big door too quickly, due to item sliding, you won't be able to press "P" and then pick up the voodoo key. Luckily, you can still right-click on the key and select "pick up".