If you've already dealt with Chikorita and Bayleef's leaves, 
Meganium should be a very easy model. Build the neck, the body, 
fore legs, hind legs and close it at the tail. Place a counterweight 
on one of the hind legs so the model can stand properly. Attach the 
antannae with superglue after you finished the head, but be careful 
cause they're fragile. When the body is done, glue the flower around 
Meganium's neck. You can leave the flower hanging like a necklace, no 
need to glue the petals' edges to the neck.