Do you have any tips on writing a resignation letter?

It is usually expected and recommended to deliver an official letter of resignation when announcing your departure to your construction employer. When writing such a letter you should try to keep things very short and positive- one to three paragraphs. This is not the forum to express your frustration or anything negative regarding your boss or the job.
Keep your reason for leaving upbeat, and general (not specific). A simple comment stating you have decided to accept another job opportunity should be sufficient. Most advisors also recommend that you do not initially give out the name of the firm where you are going to avoid any unfair attacks against the firm which may undermine your decision.
It's also a good idea to thank your boss and construction employer for the opportunity they afforded you.