I own a hotel and want to write a add letters to be attached to the guests bill and asking the guest about their stay and to tell them to check their bill attached and come settle it at their convenience, how can I do these?

Keep it short! People have a reading limit of 2 - 3 sentences for such things. Leave out the 'check your bill' because believe me, they will.
I'd say that a satisfied guest is your goal, and that all comments and suggestions are appreciated in the guest book in the lobby at the desk. I would print it on a 3 x 5 card, or notepaper sized paper.
These days, it's wise for a hotel to take the credit card information upon reservation or check in, and settle the final amount at checkout, because there are people who skip out without paying. No need for apologies about that either.