My very favorite resource for Fair Isle Knitting is called The Art of Fair Isle Knitting by Ann Feitelson. I had checked this book out from the library so many times that I decided it was finally time to purchase my own copy.
The Art of Fair Isle Knitting also has excellent explanations on armhole shaping and steeking. If you are interested in fair isle knitting I would encourage you to take a look at this book. That being said I have found that, like the Elizabeth Zimmermann books, I learned to appreciate this book more and more as I progressed in my knitting experience.
After two repeats (2 motifs equals one repeat) I got to the length where I wanted my armhole. I knit the right front sts in pattern and then placed 8 sts on a holder for the right underarm. I then cast on my 10 steek sts with backward loops. Continuing in pattern around the back I got to the left armhole point and put those 8 sts on a holder. I cast on my steek sts for the left underarm and continued in pattern across the left front.