I generally use a twin needle but that zigzag looks good so I might try it on my next t-shirt. 

I shorten t-shirts by turning the hem up, stitching about an inch from the edge but don’t cut off the surplus until after you have sewed it, as this gives a flat hem.

I managed to revisit the waistband increase, and sew down the other side of the wedge. I haven't done the return on the top yet.

  [ You are not allowed to view attachments ]  

I'm reasonably happy with most of it, but at the apex there are some visible stitches. I'm not sure if the tension wasn't enough, or in I just need to do some (bah, forgot the name) kind of figure eight stitches between the two seams, to pull them together a bit. Feel free to fill in my mental blanks, or throw forth other ideas which might tidy up that bottom end a bit.

Still, not bad for a first attempt.

When I hem the extensions to my son's t-shirts I first overlock the edge then turn it under and top stitch using a very narrow zig zag stitch - it almost looks like a straight line. As his t-shirts are mainly for work I don't bother with the double row of stitching. A good thread colour match is a good idea though.

I bought a t shirt online, that is way too long, making it look like a nightie( not a good look!) I need to cut about 4” off and hem it. I’m mainly a quilter and craft sewer and although I have made clothes way back in the past and scrubs during Covid, I’ve not really used stretchy fabric.
 Does anyone have a Janome machine, who can advise which stitches are best? I’ve googled, but can’t find anything too clear. I’ve got a TXL607. I’m guessing that I’ll need to finish the edge and then turn it up and hem.

Looks tidy compared to mine @Bodgeitandscarper. I have so much dumped in there now I have to step over stuff to get to the chair- which is piled up