Here's the sequence of what to do after you've picked up all the items and already put the skull and the doll inside the juju bag:

1. On the way to the helium tank room, make sure the balloon or a glove (whichever item you picked up first) is the current item.
2. Use the balloon and the gloves with the helium tank.
3. Walk to the coin change and, while Guybrush is walking, scroll the inventory and make the hankie the active item.
4. Push or use the coin change.
5. Once LeChuck appears, hold the right-click on him and choose "pick up". Immediately press both mouse buttons to use StopManip. Guybrush will remove LeChuck's underwear from a distance.
6. After the cutscene, keep the cursor on LeChuck, open the inventory, press Enter to select the hankie, close the inventory, and click on LeChuck.
7. After you're teleported, while walking, open the inventory, press enter to select the hankie (it will still be the current item) and click on the juju bag.
8. If you've got enough time, scroll the inventory to the bottom and use the underwear with the juju bag before getting inside the elevator. If you were lucky with the warp, do this inside the elevator.
9. Get the beard and make it the current item.
10. When LeChuck appears, open the inventory, press Enter to select the beard, and click on the juju bag.
11. Open the inventory, click on the syringe (or the doll; whichever item you picked up first), press the down arrow key and press Enter.