1. Open the config.h files in the src folder. In line 9, change the keystone index to the item index of your liking. Player has to posses that item in order to mega evolve. Build file after the change.
2. In G3T, open the 'pokemon editor.ini' file in the Customization folder.
3. Paste this in the evolution conditions section.:
Code:
FB=MegaEvo
FC=Fervent
FD=Primal
FF=RevertMegas
4. Now, open the item editor. Choose an item of your choice to be used as a mega stone. Set its Special Value 1/2 (or Held Item Effect) to 139 (in decimal) or 0x8B (in hex).
5. In the extra section of item, put the index of the target Mega Pokemon (e.g index of Mega Scizor if want your item to behave like Scizorite). Note that if you're using Kurapika's Gen3Tools you'll have to insert the ID in hex.
6. Now, open the pokemon editor, choose the mon you want to mega evolve. Then go to the evolution tab and add an entry there by setting 'Condition To Evolve' as 'Mega evolution' and
'Evolve To' to the Mega Species (e.g Mega Scizor in case of Scizor).
7. Make sure to set back the original species as Revert Megas evolution of mega species. (Unless you want your pokemon to stay as mega forever
)
(e.g Scizor as Revert Megas evolution of Mega Scizor).
8.To activate it in a battle, press the start button while choosing a move. You will hear a sound and the evolution happens shortly after.