How does a wurmple evolve into a cascoon?

A Wurmple will evolve into either a Cascoon or a Silcoon at level 7. Then at level 10, it will evolve into a Dustox (Cascoon) or Beautifly (Silcoon).

A wurmple will evolve in either cascoon or silcoon depending on its personality value.

Once you catch a wurmple what it will evolve into has already been set in stone, nothing you can do will change it. Personality values are also not accessible by normal means and are unchangeable, no other stats affect the evolution and really the only way to know what you get is by evolving it. Luckily level 7 isn't a very difficult level to reach and wurmple's are very common and easy to catch, so if you evolve one (or two or three or four) and don't get what you want just keep catching them and trying again