Instructions
Find a female Burmy. Slather honey onto one or more of the golden-colored trees in the Sinnoh region from the Valley Windworks onward.
Allow some time for a Pokemon to become attracted to the scent of honey. This usually takes around 12 hours. After a while the tree will begin to shake, indicating that a Pokemon is inside.
Press the "A" button while facing the tree to begin a battle with a wild Pokemon. There are several Pokemon that can appear in this situation such as Combee or Aipom, so it may take some time and dedication before a Burmy appears.
Catch the Burmy in a PokeBall. Lower its health or put it to sleep to make it easier to catch. Make sure that you capture a female Burmy in order to get Wormadam.
Train your Burmy to Level 20. Battle with trainers instead of wild Pokemon as these battles will give more experience points. Put Burmy at the top of your party so that she levels up faster and give her the Exp Share item to hold. This will give Burmy experience points even if she does not enter the battle.
Keep Burmy from fainting to allow it to evolve at the end of a battle. Your Burmy will evolve into Wormadam, its type being dependent on the cloak that Burmy wore in its last battle.