Finishing a sweater

I just finished the Baby Yoda sweater from KPC. The pieces look good, and I’m ready to put them together. I have no idea how! The pattern instructions just say to sew them together - should I use yarn or thread? What stitch should I use (I was thinking mattress)? There are no clear armholes, they just are set in to the sides of the sweater. Are there any guidelines for placement? Help!

Always use yarn and mattress stitch is the most preferred method for invisible seams.

Seaming links
Scroll down to finishing/seaming section.
Mattress Stitch

And check here for more.
Knitty - seaming

Thank you, thank you, thank you! I will block the pieces tonight. Will the blocking keep the egdes of the stockinette from curling up? That drives me crazy!

Stockinette always curls, it’s just the nature of the beast. Blocking may help, but it won’t cure. Check the top of this page for a sticky note about curling stockinette.

Stockinette curls, but when it’s a sweater the edges get seamed together and that will stop the curling. It’s only with scarves or wraps that it’s a big problem.

sue