FAQ and technical advice for the Celtic Cap
Working the double increase (knit into the front and back of the next stitch, pick up and knit into the vertical strand)
Clear instructions are available here.
If you are unfamiliar with the technique of opening and closing cable loops, and in particular, the centred double increase that begins a cable loop, there are a couple of books that also describe the technique:
Aran Knitting by Alice Starmore (Interweave Press, 1997): this book contains illustrations of the technique. This book is out of print. Check your local library (or use interlibrary loan), or knitting shop.
A Third Treasury of Knitting Patterns by Barbara Walker (Schoolhouse Press). This book is currently available, and provides instructions for creating cable loops. The directions are slightly different than the instructions given in the Rogue pattern.