How do you get the 3 forms of Wormadam?

Okay, when you have all your female Burmys, they evolve into their different forms depending on where you leveled them up to Level 20.

Wormadam (Plant cloak) - any grass route.
Wormadam (Sandy cloak) - any cave or sand patch.
Wormadam (Trash cloak) - any indoor area (use the restaurant near Lake Valor or the Elite Four for this).

Hoped I helped.

First of all,your Burmys must be female;they will otherwise evolve into Mothim.Next,level a Burmy up at one of these locations to evolve them into the corresponding form:

Plant Cloak:In an area with grass patches.(Ex.Pokemon Mansion patch0
Sand Cloak:In an area such as a cave(Ex. Mt. Coronet)or a mountain(Ex. Stark Mountain interior)
Trash Cloak:In an area inside a building(Ex. Trainers inside Pokemon Centers,Lake Valor restaurant,etc.)

Hope this helps!

Go catch 3 female burmys then you need to train them up to level 20 in certain areas:
1. The grass cloak: Train up Burmy in a grassy area like a route.
2. The sand cloak: Train up Burmy in a desert or cave like ravaged path.
3. The trash cloak: Train up Burmy by battling trainers in a building.
Quite complicating at first ;)

Catch 3 female Burmy from Honey Trees and it depends on where you train them to evolve into the certain forms.

Grass Cloak: Grassy Patches

Sandy Cloak: Caves

Trash Cloak: Indoors

There's two things I don't get though: What happens when you evolve Burmy in water? And since Burmy has the same three forms can, let's say, for an example, A Grass Cloak evolve into a Sandy Cloak?