A lot of ditzy celebrities are desirable, such as Jessica Simpson, Marilyn Monroe, Britney Spears and Paris Hilton.

[edit] Steps

Appearance- Dye your hair blonde, so you can fit the stereotype of a "dumb blonde", wear light colors like pink white and yellow. Look innocent and spacious.
Actions - When talking to someone tilt your head slightly to the side, open you mouth slightly so you would look like you don't understand, but also nod your head as they speak as if you’re "trying" to understand. Look away and act like you were distracted by something; your eyes should be wide open when they are talking to you, you have to look empty brained and spacious. Play with your hair.
Speech- When you’re speaking, have a slightly high pitched voice but also be slightly breathy. Talk slightly fast, and try not to make sense of what you are saying, repeat what you have said twice in one sentence. When someone asks you a question answer incorrectly and when they correct you simply open your eyes even wider and say "Oh", then bite your lower lip, look to the side and shrug your shoulders.
Laugh- Laugh at silly things, make sure your laugh is slightly high pitched, Do not laugh when you have answered something incorrectly because that will show that your answer was planned.