How do I "sew up the sides and then weave in loose ends"?

I picked up a Plymouth Yarn Design Studio project while cleaning out a closet, realized that I finished the knitting a long time ago, but never finished the project, a vest. I need to sew it together (and weave in the loose ends) but have never done that before. Any instruction would be appreciated.
Thanks in advance.

I line up what needs to be where on the pattern and sew it together with yarn, kinda like basteing it. When you get the stitches lined up sew them together but only go into half the stitch or it will show on the other side. When that is done you just weave the short tail thru stitches till its gone. No need to do alot maybe 5 or 6 inches into it . I go up and then down so the stitches done come undone. Good luck!

The best way to seam the sides of a garment is mattress stitch. Shoulders are often done with a 3 needle bind off for stability. Here’s some help videos and tutorials.

Mattress Stitch - scroll down to finishing in the first link

I weave in ends like this -

There are 3 needle bind offs on those first two links. If you’ve already bound off the shoulders here are some other techniques -

I appreciate the video referral. It helps me to watch someone else do it.

Thanks for the tip.