Above are images of Knitting Jenny Pattern 31, a Fair Isle fisherman's kep hat that I designed when I had a home and studio in Fair Isle, Scotland last year.
The photograph on the left is the knitting front, and the photograph on the right is the knitting back. Note the floating yarns on the knitting back.
My Fair Isle inspired knitting patterns require the stranded colorwork technique of floating yarns.
Floating yarns are defined as the wool or yarn that a knitter carries along the back of the knitting when that wool or yarn is not in use.
For accessories for the head and hand, carry the yarn (that is not being knit) on the inside of the hat or mitten, and ensure to carry the yarn neither too tightly nor too loosely.
For Shetland wool with which I most often knit, I generally float a maximum of one inch (without catching). Floats of more than one inch might need to be caught. Many people choose to catch floats more often.
If there are extremely long floats inside the hat or mitten, then the tension might be affected negatively. Also, there is a risk that fingers might get caught on (or snag) the extremely long floats when placing the hat onto the head or when pulling the mitten onto the hand.
To avoid extremely long floats, periodically catch the yarn on the inside of the knitting by twisting with the opposite color.
Stagger the catch position on each row so the catches are randomly arranged. Ensure that the catches do not line up vertically, systematically one upon the other, which can unfortunately become visually apparent in the finished knitting.
Alternately, carry extremely long floats across the back of the knitting without catching. Then, when knitting the next row, pick up the extremely long float and knit it together with a stitch from the current row that is being knit.
Lastly, you can opt to forego catching floats altogether. The nature of Shetland wool ensures that during the blocking process, the floats generally stick into place. That is a beauty of Shetland wool: it intuitively does what you need it to do!
Whether to catch floats, when or how, is entirely a personal knitting preference. There is no one way. I encourage experimentation to identify which knitting method is best for you. Good luck, and happy knitting!