To use this trick, you first must have a full party before starting the battle with the Professor. After defeating him, you can select your starter, and the Professor will say to send a Pokemon to your box to make room for your starter. Tell him "No" (or press B), and he will hold on to your chosen starter until you decide to collect it. He will then continue with the normal conversation. After gaining control of you character, you can save and ask the Professor for your starter. You can then select a Pokemon to send to the box, and he will give you the one you chose. If you do not like the nature of it, soft reset Pokemon X/Y (press L + R + Start + Select). Once done, load your saved game, select your box Pokemon, and scroll through a couple lines of text to be able to check the nature of your new starter. Repeat this trick until you get an Adamant Charmander or the desired nature for your starter Pokemon. Note: You cannot select a different starter after using this trick. The Professor will always be holding onto the Pokemon you previously chose.