If you knit, you might be familiar with the mattress stitch. In knitting, mattress stitches are used to vertically join two pieces of knitting in an invisible way. A slightly different variation on the mattress stitch can also be used to invisibly join pin-loom pieces together. As an added bonus, unlike joining pin-loom pieces on the loom (as I did in this post) your fabric won't have a "wrong" side.
In the illustration above you can see the basics of the mattress stitch when used on a pin-loom square. Notice that the squares are positions so that the "bumps" and "dimples" are offset with the bump on one square sitting in the dimple of the other. When you're setting up your pin-loom pieces for mattress stitch make sure they're offset like this from the get-go.
Next, thread your seaming yarn on a tapestry needle. Pass the threaded needle through the corner loop of both layers from front to back, then pass through again to secure.
Pass from the wrong side (inside) to the right side (outside) of one square through one half of an edge loop, then pass back to the wrong side through the second half of the loop. Repeat with the other square, moving back and forth between the squares in a zigzag.
While the mattress stitch is relatively simple, I do have a couple quick tips. First, you can use the tails leftover from weaving your squares to join pieces together—just don't forget and accidentally weave them in before joining. Second, if you plan on joining many pieces with mattress stitch, where you'll be joining tops, sides, and bottoms, lay out your squares first on a towel or a big piece of felt. Make sure that all the bumps and dimples are offset in a way that gives you the final shape you want in the finished piece.
Happy Weaving! Christina