I’m making a patchwork lap blanket and the pattern says to make 20 8in squares in garter stitch. Then it says to sew them together with contrasting color yarn. I have no problem sewing them, but my question is, how do you sew them together without the stitches falling out? Do you knot the yarn? Do you sew left to right and right to left again? What is the best method?
how do you sew them together without the stitches falling out?
Not sure what you mean by ‘falling out.’
Assuming you’ve established good selvage sts on the right/left edges and uniform BO/CO you can use mattress st. If you crochet, you could border each sq in sc and then crochet them tog. I don’t think a whip stitch would work particularly well on garter. Whatever st you choose it needs to stretch consistent with the sqs. You don’t want it so tight that the sqs warp and not so loose that the sqs tend to gap and look ‘swimmy.’ You may need to test out a few diff options on a couple sqs to see which works best for your tension.
cam
Well, I am a beginner and I didn’t know if my sewing stitches would fall out and the blanket would fall apart.
But what you’ve said has made some sense and I will try out all the things you said. 
Once you start seaming/sewing into the edges your yarn will be secure so long as you pick up the sts correctly (picking up a loop on side of one sq, picking up a loop on the other).
Assuming you did a test swatch of your yarn, you could make another, seam them in the manner you think looks best, make sure to secure the ends (remember, this is just a test) and do the wash/dry thing. See how it looks afterwards to determine if it held up and if you feel the need to make chgs. If you’re uncertain as to the durability, best to find out on a couple of sqs than to sew the entire thing and then discover your seaming choice wasn’t correct.
cam
Here are couple examples of seamed sqs:
I presume Jints used mattress here.
I used crochet here.
cam
On all over garter stitch, mattress stitch would be good.
However, like you ‘Cam90066’, I actually crocheted those squares together but from the wrong side. Good result. I tried the mattress stitch on that one but had trouble getting a tidy uniform seam with all the different block sts.
Thanks for the clarification, Jints. Your having worked the WS made for such a clean finish on the front.
cam
Thanks everyone! 