1. Catch yourself a Ditto. You may want to get 3 so you can clone up to 3
different items in one battle.
2. Get yourself an amount of Pokemon with a stealing item move that equals your
Ditto. Moves that can steal items are Trick, Covet, Thief and Switcheroo. I
personally like Kecleon, as they are slower than Ditto and can learn Thief by
level if you can spare a Heart Scale.
3. Find yourself a double battle. I personally like the one below Floaroma Town
with the two twins.
4. Position your Ditto in Slot 1, your "thiever" in Slot 2, or vice-versa. Put
the rest in any other slot if you brought more.
5. Use the VS Seeker and hope the double battlers want to battle you. If they
do, engage in battle. Make sure your Ditto are holding items, and your thievers
aren't.
6. You should send out Ditto and your thiever. If your thiever is faster than
Ditto, take out one of the opponent's Pokemon and have Ditto transform into any
Pokemon on the field. If your thiever is slower than Ditto, then have Ditto
transform into any Pokemon on the field and your thiever attack Ditto with the
thieving move. You should steal the item. Obviously if your thiever was faster,
you use the thieving move on Ditto on the second turn rather than the first.
7. Check the switch menu. If your Ditto and your thiever both have an item then
you have preformed the glitch correctly. If not, then something's gone wrong or
it just doesn't work on your verison of the game.
8. If you wish to continue, switch to your second Ditto and second thiever,
repeat step 6 and 7, and do the same for the third Ditto and third thiever if
you have them.
9. Finish off the opponents.