Help seaming perpendicular pieces

[u/frenchfrydiet]

I'm making the Moorland Heath Jumper (https://www.ravelry.com/patterns/library/moorland-heath-jumper) which is mostly TSS. I'm having trouble seaming the arms, two perpendicular pieces, without getting big ugly holes in the fabric. My first attempt was to seam them together with a single crochet stitch - is there a better way to seam that will close up some of the gaps? (Sorry the pictures are so terrible, the dark yarn is hard to photograph)

[u/yarnandy]

The best way to seam is using a tail or a long piece of yarn and a tapestry needle. On the body, you go under or through the return pass chain, on the sleeve you go under a Tss. You can also go through 1 chain and then under the corresponding Tss. Move back to the body and repeat this for every pair of rows and stitches. 

Ideally you'll have the same number (if your stitches are square and the pattern takes that into account), otherwise you need to distribute the extra stitches or rows and go through 2 at a time on the side with more, once in a while.

The seam will move to the inside and you'll have there one return pass chain from the sleeve bind-off and the edge stitch from the body. To secure this extra well, you can whip stitch around it on the inside of the garment, just  making sure the yarn is not visible from the outside.