Write a resignation letter to your manager to let him know that you have started to cooperate with another company. In your letter:
•let him know the exact time that you are leaving
• the actions you took in order not to cause the company any problems
• and also let him know that you appreciate the time you cooperated with the company.​

Dear Mr. Amish!

I would like to inform you that in two weeks' time I am leaving your company. I found a place where my talents will be appreciated. But nevertheless I am grateful to you and your company for the experience I have received.

Your company will not suffer any changes as I have prepared Mr. Smith for my place.

Best regards, Mr. Hamish