Welcome to the Willow Poncho seaming tutorial. This seam uses the Mattress Stitch, and while this tutorial is not specifically about how to do the Mattress Stitch, it is about how to grab the stitches when your edges contain both stockinette stitches and garter stitches.
If you’re not familiar with the Mattress Stitch, this link will take you to your choice of Mattress Stitch video tutorials.
To create this seam, use your tapestry needle to run the yarn under the bars between the stitches on the stockinette edges and under the outermost loops on the garter stitch edges. Sometimes you have to dig around a little to find the bar between the stitches on the stockinette edge. Keep trying, though; it’s in there!
Work flat, on a table, keeping the two sides even. Snug up the yarn as you go – but not too tightly.