The easiest way to cook fresh hard-shell littleneck clams for chowder: (steam method):
Discard any clams that remain open when tapped with fingers. Scrub clams with stiff brush under cold running water. Soak clams in mixture of 1/3 cup salt to 1 gallon water for 20 minutes. Drain water; repeat 2 more times. Place 1 cup water in large stockpot. Bring to a boil over high heat. Add clams. Cover stockpot; reduce heat to medium. Steam 5 to 7 minutes or until clams open. Remove from stockpot with tongs. Discard any clams that remain unopened. Cut steamed clams out of shells with utility knife. Place into a small bowl, cover, and chill until needed. Add to pot at the end, the same as for canned, along with a small bottle (8 oz.) of clam juice if desired.