Sewing up a side seam in knitting

I used my cast on yarn to start sewing my side seam of my sweater. I don’t have enough yarn. Can someone tell me how to add additonal yarn to sew up the seam please. Should I tie a knot and continue that way?
Thank you

As always, I’ll defer to the experts when they chime in, but…

If your tail from cast on was too short to sew up the entire seam, I’d weave in that end and finish seaming with a new length of yarn starting a few stitches back for a little bit of overlap if I could.

Mattress stitch is the best way to seam the sides. Just cut a long enough length of yarn…I’m not sure of the length to seam ratio so be generous and then leaving a 6 in tail do the seam. You can weave in the ends later. No need to knot.

Scroll down till you get to finishing techniques.

This is the method I use for weaving in the ends.

I agree with justplaincharlotte!

You don’t have to use similar yarn to the project when you used the mattress seam
because the seaming yarn doesn’t show when it’s all done. It becomes buried in the seam.