Regarding grammar, spelling, and punctuation, all should be correct. However, I believe you are asking more about the proper protocol than the proper grammar. Here is my suggestion. A letter of resignation should be to the point and should express a reason for the resignation. It should not be the real reason, if the real reason offends. This is because there could come a time down the road where you would seek either a letter of recommendation or a position with the same company. So you want to leave on good terms. In brief, the format for a letter of resignation is one that does not burn bridges. By framing it this way, you make the situation work to your advantage.