Shopping The Mortgage

How and when do I "shop" a mortgage? (first-time buyer questions inside) So, I've got a lender I'd like to work with (found via referral). But I want to make sure that I'm getting a fair deal, so I'd like to "shop around" and see that the rate/closing costs/points/etc that I'm getting are competitive. But I'm not sure when or how i should shop the mortgage. Do I call and compare before i've found the home, after my offer has been accepted / entering contract, or what? Basically, I'm looking for a timeline of the loan process when buying a home with regards to shopping lendors. And how does "locking in" a loan figure into this?