I would like to know when you could send me the letter.
As soon as I receive the letter, I will be able to schedule an appointment.
The second sentence can be changed to many things. It just depends on what you're trying to say. Will you be able to schedule the appointment after you receive the letter? Or will you defiantly schedule the appointment after you receive the letter? If you will schedule the appointment, then say you will. If you're saying that you be able to, then say that. Overall, good sentence. You did well.