Doubling over your roll neck is super simple just a bit fiddly. Once you’ve knit to the correct length you need to pick up your cast on stitches one by one and place them on your left hand needle. You then put your right hand needle through both your original stitch (the one that you’d have knit if you were starting a new row) AND the cast on stitch that you’ve placed on the needle. You knit the two stitches together to effectively join your 2 rows.
Carry on repeating this process until you’ve knit all your cast on stitches in with your regular stitches, making sure you’re aligning them correctly so if you’re onto your 4th stitch, make sure you’ve picked up the 4th stitch on the cast on row also – if you’re knitting a rib it makes it easier to check you’re all in line as you can follow the rib round.
If you are knitting a rib don’t forget to carry on your stitch pattern, for instance, if you’ve been working in single rib [K1, P1] you’ll continue to alternate between knit and purl….knit the first stitches together then purl the next stitches together.