How do you write a letter to your friend about your teacher?

The same way you write any other letter.

An opening salutation, a greeting, the main body of the text covering the subject and conveying the information you wish to impart.

Any other information you wish to pass on.

A closing salutation.

Use an envelope of a suitable size, place the letter inside and seal.

Address the envelope to your friend. Buy suitable postage and post in appropriate receptacle.