How do you transform into a girl?

I will assume you mean to transform into a girl from a boy. One one level, it is not possible. Feeling like you are a girl or a boy is inborn. So it is possible that despite whatever body you have now, you are already a girl.

Now, if you mean socially, physically, and or legally, here is how.

Socially:

Start dressing as a girl.

Start acting like a girl.

Learn how to speak like a girl.

Do activities that girls do.

Hang around with girls.

Physically:

See a gender therapist and get letters for hormones and surgery.

Take female hormones.

Get sex-reassignment surgery.

Legally and fiscally:

Petition the court for a legal name change.

If you attend school, register under the female name.

Get a letter from a physician saying you are a girl.

Apply for a new or amended birth certificate with the correct name/gender.

Use the amended birth records to change your social security records.

Use the amended birth records to get a new ID or license.

Get all bank and credit accounts in your female name.