The first step in the process of making a monthly budget for a mortgage application is to determine your income. Find your tax returns for the last three years, five would be even better. You are almost certainly going to need at least two years’ returns for your lender anyway.
Recent paycheck stubs will also be needed, along with W-2 statements for the past few years showing your yearly income. If you are salaried, look at your gross income and the changes over the years. This should show you (and your lender) if your income steadily increased, decreased, or was relatively stable.
Stable means that it kept up with inflation. If there was no increase or an increase less than inflation over the years, it will be considered to have decreased.