Set up a meeting with your mortgage broker. Have all financial documents available, and have the lender prequalify you based on income and monthly debt payments. Discuss your plan to purchase the home below appraised value. He will make recommendations early on to prevent you from having to come up with a larger down payment than you intended. He will also discuss costs to close the loan, and escrow funds for taxes and insurance that you will need to have in advance of closing.
Talk to the lender about credit and scores. Pull your own reports in advance by going to www.annualcreditreport.com to see if they show any negative information. Do this early to have time to dispute and resolve issues.
Begin your loan process, but prior to executing a contract on the targeted home, have your real estate agent do a "sold search" in the neighborhood to see what other homes have sold for. Get the printed list from the realtor and drive by these sold homes to see how they compare to the one you are interested in. If this information is not sufficient to assess value, talk with an appraiser.
Call a local appraiser and schedule an appoint to talk with her. Understand that if you decide to have her do an appraisal on the home, the lender probably will not be able to use it since new rules in how appraisals are ordered now apply. Remember, this visit (and possible extra small cost) may save you much more in down payment money at closing.
Stay in touch with your lender without a contract. When you have sufficient information on value (either by "sold search" or appraisal), you will feel more secure in making a contract offer.