I posted the picture above on Instagram and Flickr and a lot of people said they wanted to try Clamshells too, so I thought I'd whip up a little tutorial so you can :)
You will need:
Squares of fabric
a strip of fabric approx 8" wide x however long you want your finished project to be (mine is about 20" long)
Some squared paper
Plantpot/mug/bowl/something round to draw around I used a plantpot that is 4" in diameter
Needle, thread, scissors, iron
To make my Clamshells I am using a 5" charm pack. I chose Marmalade by Bonnie and Camille for Moda because it is summery and cheerful, but you could use scraps if you like.
Making a clamshell template is a lot easier than you think. Place your round object on the squared paper and try to centre it on the squares. Draw round it and then mark 12 o'clock, 3 o'clock, 6 o'clock and 9 o'clock. Use your markings to draw a circle the intersects at 3 o'clock and 6 o'clock, and another that intersects at 6 o'clock and 9 o'clock. This gives you your clamshell template without seam allowances.
Cut your template out and transfer it to cardboard.
Place your cardboard template on a charm square and draw round it using pencil (or whatever you prefer). You can mark your seam allowances if you wish, but I just eyeballed a 1/2" seam allowance and cut it out.
Take a piece of kitchen foil that is slightly larger than the cut clamshell, and place the cut clamshell on top, followed by the cardboard template. Carefully use the foil to fold the top curved edge of the clamshell around the cardboard template. Press with a hot iron. Allow to cool and gently remove the foil. You should have a lovely smoothly curved seam. The clamshell is now ready to be appliqued.
For the top row of clamshells I placed 4 clams along my fabric strip, making sure they were all level, with the sides just touching but not overlapping. (Sorry I hadn't started taking pictures at this point!) Pin them to the background strip and applique the top curved edge only. I hand stitched using small invisible(ish!) stitches, but you could topstitch if you prefer.
Once your top row is stitched in place you can begin layering the clamshells one row at a time, pinning and stitching the same way as the first row. You could go as big as you like! I'm planning an 18" cushion. When you layer each clam make sure the 12 o'clock position is lining up with the meeting point of the 2 clamshells directly above it.
Here is how it looks from the back to give you a better idea
Hope all that makes sense. If there are any questions, please give me a shout.