3.Find an online mortgage calculator and estimate what the monthly payment would be for the property you chose. First, multiply the price of the home you want to buy by 0.1 to calculate the 10 percent down payment you will need to obtain a mortgage. What is the 10 percent down payment amount? Next, subtract the down payment amount from the cost of the property. The result is the amount of money you will need to borrow from the bank. What is the amount? Enter this figure in the online loan calculator as the loan amount.