Once you have documented all your income sources, it is a good time to firm up any promises for help.
A gift letter is a letter stating that you are receiving money for part of your down payment as a gift. The letter states the amount and that you have no obligation to repay it. It is not a loan.
Now is the time to contact that uncle who said that if you ever need money for a down payment on a home, he would be glad to help. If he was serious, draw up a gift letter and have him sign it.