A Fair Isle chart is so repetetive that they're not difficult. You mark off everything above the row you're working on. Usually, the entire row will be, for example, 2 blue, 3 white--all the way across.
Or 4 blue, 3 white, 1 blue, 3 white, 4 blue--a balanced repeat pattern.
As you make a design, you watch it develop. If you have a design that's shaped like an X, you're going to start with the bottom part of the X. On the next row the colors will move in as the bottom of the X moves in. After a while, you can see what's supposed to happen so you don't make a mistake.
Also, with a chart, you can see what was on the row before, so if you've got a blue that's supposed to go above a blue on the chart, and it's going above a white, you know you made a mistake. The only time I really screw up is on the first row. Once I know that's right, usually after a few tries :rollseyes: , each row is used as a reference point for the next row.
Complicated intarsia requires more attention to the chart, but I usually will look and say, "ok, three red--1,2,3. Next, 5 blue, 1,2,3,4,6 and just read each part at a time. If I lose my place, the knitting is on the needle to show me where I'm at.