Dont bother about format so much just take reference from ACS site..make sure all your details and supervisor/senior collegue details are mentioned along with detailed roles and responsibilties.
I went with 100 rs stamp paper from senior collegue and it went fine.
You can also write your own wording by following any company ref letter.
I have no access to attach here it seems.
Do.let me know i will type for you some template if u want else you search you should find here on the forum already.