I use welts (also known as tucks) in several of my patterns, including my most recent, Capall Dubh.
Welts are typically over 4 rows but you can very the size of them to suit your needs.
To begin this welt you start with 4 rows of stockinette stitch, finishing with a wrong-side row.
In this example I’ve used a double pointed needle to pick up all the purl bumps from the row of stitches 4 rows down. If helps if the needle size is smaller than you are working with as it will easily slide into the stitches. The first and last stitches on each end will just be the loops right at the edges of the work rather than full stitches. Count your stitches before you begin the welt to ensure you have the same number of stitches picked up as you have on your needle.
Each of these stitches can also be picked up one at a time as you work but I think it’s easier to get accurate results if all the stitches are picked up together.
Knit one stitch from the front needle together with the back needle, this will join the two rows together to form the welt.As you are working across the stitches this is what your welt will look like.