For most seams I really like to use mattress stitch which is also sometimes called invisible seam, running thread seam, ladder stich, vertical grafting and weaving. With mattress stitch, the seam is generally sewn one stitch in from the edges thus requiring a one-stitch selvedge on each edge.
For the kangaroo pocket on the front of the Ruby hoodie, the principle is still the same, but the pocket is sewn onto the front several stitches in from the edge.
The Ruby pocket also has an extra stitch on each side specifically for the selvedge.
Position your knitting so the right sides are facing you and thread the tail of yarn onto a tapestry needle. If the tail’s too short then use a fresh length of yarn.
On the sweater front, insert the tapestry needle under the bars between two columns of stitches. You can pick up one or two bars at a time, but never more than two bars. Pull the yarn through loosely.
If seaming two selvedge edges together, you should be picking up the bars in the ditch between the first and second columns of stitches.
Insert the tapestry needle between the first and second stitch on the edge of the pocket and pick up one or two bars. Pull the yarn through loosely.
Note that when your two edges are the same length (same number of rows) you should consistently pick up the same number of bars between stitches on each side so they seam together evenly.
After working a few loose stitches on each side, pull the sewing yarn to take up the slack.
You don’t want to pull so tightly that the fabric puckers, just enough for the two sides to come together and the sewing stitches disappear into the fabric.
When the seam is finished, pull the sewing yarn through to the back and weave in the end before trimming the excess yarn.