To avoid as many sewn seams as possible, I like join the top of the kangaroo pocket to the Ruby hoodie front by knitting them together.
Since writing this tutorial I’ve used this technique for joining two fabrics together on other projects, too.
This technique is very similar to a three needle bind off, without the binding off part.
Knit across the sweater front row to the point where they are to be joined.
With the wrong side of the pocket front facing the right side of the sweater front, hold the two needles parallel.
Insert the right needle (the one with the stitches from the first part of the row for the sweater front) into the first stitch on both needles.
Wrap the yarn around the needle and knit both of those stitches together.
Repeat Step 2, joining the next stitch from the pocket and the sweater front together until you run out of pocket stitches.
Knit to the end of the row working on just the sweater front stitches.
Continue working sweater front.
Check out the tutorial on seaming the side seams of the pocket.