The instructions are for circular knitting where every round is knitted. But this works just as well for flat knitting with knitted and purl rows.
The two-loop join
The two-loop join used for the new background colour
Use this join every time you need to change the colour of the main colour for Fair Isle.
The sliding loop join
The sliding loop join used for the new contrast colour
The two-loop join (again)
At the end of this small Fair Isle sample, the main colour is changed again. Use the two-loop join as above.
The Fair Isle sample was made with: