1. Valid contract (contract signed by buyer and seller) and all addenda.
2. Last three months bank statements.
3. Last year tax return, or wage and tax form (W-2 form).
4. Last two months pay stubs.
5. All loans you applied. Such as home loan, auto loan or student loan.
6. If you are a self employee, last two years tax returns and current gain and lost report.
7. If you are renting a house, evidences of rental payment, such as rental payment receipts, check copies or payment letter written by rental company,  are required.
8. If you have a rental house and have tenants, rental contract is required to prove your rental income.

Usually, on the contract, buyer is required to apply a loan within certain period after under contract. So once under contract, buyer need to contact a loan broker/mortgage lender to applying a loan as soon as possible.

Mortgage rate usually changes everyday and buyer should work with loan broker/mortgage lender closely to ensure get a good current mortgage rate. Ask your loan broker/mortgage lender to keep you posted with current mortgage rate and lock the rate 30-45 days before closing.

You need to know that loan broker is different that lender. Lender is a financial organization like bank. Loan broker is an agent representing you to dealing with lender to get your mortgage. You don't have to choose a loan broker, instead, you can directly contact mortgage lender to apply a loan. However, you may not get best mortgage rate.