How to write a cancellation letter?

And send it return receipt requested. Never mind that you just asked for an acknowledgement; this is proof that you actually sent the letter, and when.

If you owe money, they will bill you. And they will probably try to get you to come back. Keep telling them no.

Sign and date the letter and add whatever identifying information is necessary for the timeshare, e. g. the timeshare's address or your account number or the like, and for God's sake don't put your email address or phone number on the letter. They are going to try to get you back (and they might have that information already anyway), but you don't need to make it easier for them to bother you.

Is this a writing assignment?

I don't believe you "cancel" a timeshare. That is a legal document and there will be a buyout or transfer of credits on your account.