Pocket lining made with add'l stitches on spare needle

Either I’m losing my mind or I just can’t find any instructions that make sense to me in order to follow these pattern directions:
Make pocket: work 17 stitches in pattern. Place a marker. * With spare needle held in back (!!!) make lining sts by inserting right needle into next st on left needle. Pull through a loop and place on spare needle. Knit into back loop of st on left needle * (That’s the confusing part). Repeat 30 times more; attach 2nd ball of yarn, working 1st and last st of lining in garter st and remainder in stst until lining measures 5 inches.
I am confused about how to create the lining stitches on the spare needle, and also about whether the instructions to knit into the back loop of the st on the left needle are referring to the new stitches or the existing (pattern) stitches remaining on the original left needle.
Can anyone out there help me understand these instructions? I’d be forever grateful!!!

Essentially you need to knit to the indicated stitch.
Then use the 1st stitch on the left needle to do a knit CO onto to a spare needle (not your current right needle). Then use your right needle and do a Ktbl of the same left needle stitch.
Do this 30 times