HOW CAN I WRITE A GOOD LETTER TO MY BOSS?

Be honest and tell him your situation. Explain that you enjoy the job but that the current project seems to have less going on and that for that reason you don't have much work to do. If he can help you, he will find more work to do if that is what you want. Otherwise, he may just tell you to deal with the current project till it finishes and a new one comes up. I think this is a temporary situation. Make sure the letter is short and simple. Your boss is busy and will not have time for a long letter. Also let someone you trust read it so that you have written it well without errors. You don't want to have mistakes in a letter you write to your boss of course.