Geoduck, first make sure it is alive.
If pulled from a tank, it should pee water.
Next when you get home, a good scallop knife or really any sharp blade will work, just cut around and remove the 'neck' and all attached parts. I discard the stomach but reserve the mantel and any clam like parts for chowder.
The neck(siphon) is then briefly (30 seconds) dipped in boiling water and then into an ice bath to stop the cooking. Peel off the outer membrane. you now have a semi-prepped siphon.
Cut, a sashimi or super sharp filet knife is good for this, horizontally, you now have two pieces.
For traditional sushi/sashimi cut a block and then lightly score parallel to the long axis of the siphon on the outside, cut slices perpendicular to the long axis of the siphon and slam one down on your work station and watch it curl, exclaiming, "it's alive" at the same time. Old sushi Chef trick!
Slices of the neck can be dipped in corn starch, fried and served with lemon, best fried clam you will ever have.
I buy them at H-Mart when they have them.-Richard