Joining pieces

I’m making the initial bag from S’nB Nation (Letter Have It, pg. 159). I’ve completed all the pieces (thanks to help from Ingrid), but I’m not sure how to join them all together. There’s a front and back, both in stockinette, and a center panel in garter stitch. Most of the joining techniques call for lining up the pieces row for row, but I’m sure my center panel doesn’t have the same number of rows as the front and back pieces.

Thanks in advance!

Here are some different types of seams.