Imagine you are faced with a puzzle. You have two pieces of knitted garter stitch fabric and you want to sew them together side to side. How do you do it?
It's surprisingly easy. You use your knitting super powers and come to the conclusion that mattress stitch will give you a nearly invisible, but surprisingly strong way to join the two fabrics together.
Start with a long piece of yarn. I make mine three times as long as the seam I want to sew. It's best to use the same yarn that you knit the fabric with. If you don't have any left, perhaps you could find something roughly the same weight and similar colour.
Next you need a blunt (darning needle) just large enough to fit the yarn through the hole. I actually have only two blunts, one large and one small. The small one I use for socks and small yarned projects, whereas the larger one is perfect for sweaters, hats and projects with larger yarn . There is no real hard and fast rules here, you can use just about anything to pull the yarn through the loops. It is easier, however, to use something with a blunt tip as you want the yarn to flow between the stitches of the knitted fabric, not splitting through the yarn that makes up the fabric. You'll see what I mean once we get some more photos.
Lastly, some garter stitch fabric is good, also something to cut the yarn with at the end, is also quite useful.
Now for the fun part.
Lay the two pieces of fabric, Right Side Up, next to each-other, on a flat surface. Place them how you want them to be together once they are all sewn up. Take some care at this stage, it is surprisingly easy to get something backwards or inside-out. Really, this is the hardest part, so check, check and check again. When you think you have it perfect, walk away and go make yourself a cuppa tea. Once the tea is in hand, check one more time to see that the fabric is exactly how you want it. I'm not trying to be patronizing, I always mess up this part and have to un-sew it and re-sew it and all that work is annoying. Please, learn from my mistakes.
(Those with keen sight will have noticed that this fabric is actually the wrong side up. It just goes to show you how easy it is to get things backwards, up-side-down or wrong-side-up.)
Now we are ready to sew up the seam. Thread the blunt and fasten the yarn to the corner of the fabric nearest to you. You can use the left or the right fabric, but what is important is that you leave at least four inches of yarn to weave in later. I use a half hitch at this point as it is easy to undo if something goes horribly wrong yet is firm enough to hold for any knitted fabric.
Do you see how the garter stitch fabric is made up of ridges and valleys?
We are going to work with the ridges.
First put the needle through the bump nearest to the edge on the right side of the fabric.
Pull the yarn through so that it is firm but not taught. If you pull through too far, the fabric will bunch up, if you don't pull it through enough, then the seam will drift apart over time. The ideal tension is as close to the knitted fabric as you can get. This takes practice.
Now, on the left side of the fabric, do the same.
Keep on keeping on. This is all there is to it. Once you get the feel for what you are doing you can seam both sides at once as shown in this next photo. It makes it go twice as fast.
Once you get to the end of the seam, fasten the yarn with a half hitch and weave in the ends.
The seam will be almost invisible on the right side of the fabric, and will look like this on the wrong side.
Once you do it, it is surprisingly easy. In fact, it took me more time to explain how to do it than it did for me to sew up this vest.