Im no financial genius, but here is what I would do. Work backwards, figure out what the monthy payment(approximately) is going to be on your new mortgage. Any banks mortgage calculator will work. Then add to that number the rest of your monthy bills. The total needs to be around 40% of your gross monthly income. If you have good credit or very high income, it could be a little more, but 40% is a good ballpark. Now that you know what you need to prove you make, take a look at your two previous pay stubs. If the monthly gross is over your required gross, then you are set. If not, see how many hours over guarantee you would have to work to make the numbers work. If its a reasonable number, Im sure someone in payroll or HR would be willing to write a letter to that effect. Also, find a mortgage company that is willing to work with you. There are a lot of good companies out there that will walk you through the whole thing. I was in the same position when I bought my house, and it ended up being much easier than I anticipated.....granted they were handing out loans to anyone back then.

Good luck!