1. If you purchase fresh clams, you need to remove as much sand as possible. I find it’s best to first soak them in salty water for up to 24 hours before serving (so try to buy them a day in advance). When you get home, rinse & place them in a large bowl, cover with water and add 1 Tb. salt. Place in refrigerator then check back in about 12 hours (ideally 24). The clams will do their thing in salt water - open to take in water and expel any sand from their shells.
2. When ready to make chowder, remove clams from the sandy water and place them in a saucepan with 1 c. fresh water. Cover and steam a few minutes, until shells open. Remove from heat, discard unopened clams, cool a few minutes, remove meat from shells and place it in a small bowl. Pour the pan juice over the clams but hold back the last tablespoon or so since it probably contains sand. Allow the clams to rinse a bit more and the juice to settle. Remove clams, then measure out the juice but hold back the last tablespoon or so since it probably contains more grit -- it should be close to 2 cups.