Harriet Hargrave has a great method in her book Mastering Machine Applique. It's on 97-99 of the second edition. I've used it several times and have been very pleased. The clamshells are machine appliqued onto a muslin background. Maybe I'll post of picture...

There is a free pattern that might be easier for you at http://www.hoffmanfabrics.com The pattern is named KASURI HILLS. I have this in my pattern file but haven't tried it yet. Looks easy!

Kaffe Fassett in his book on quilts from the Museum (Vand A museum) makes a replica of a clamshell quilt

You could try those instructions. A couple of my friends have done a class based on that quilt and have had some success with his instructions

I have started cutting out pieces for a clamshell quilt but I hesitate in putting it together. Do I hand piece it? Do I applique the shapes to a muslin background? Do I do english paper piecing, and how? Has anyone out there tackled a clamshell quilt and how did you accomplish it? I would appreciate any insight.