She uses a technique called tapestry crochet, where you 'carry' different coloured yarns, working over the colours you're not using, and swapping to the colour you want before finishing a stitch. The instructions in her book are beautifully clear, but here's an excellent, quick demo from youtube. It's a really simple technique to master, but working with pattern and colour are a completely new field for me.
I've been lost in my sofa for the last three weeks, trialling different approaches, and here's what I've learned.
a. working in the round can give the tidiest finish, but your design will lean to one side so it's not so good for an image or more complicated design.
Working in the round is great for geometric designs.
b. Turning at the end of each row will give a slighty rougher 'edge' to your design, but works well for a bigger pattern and doesn't lean
c. Working only one way - so chopping off your yarn and starting again from the same end each time means your design doesn't slope, you get a nice tidy finish, but the work itself leans to one side (and you have a lot of ends to weave in).
The top piece is worked turning at the end of each row,
the bottom piece is worked only one way.
To get a lovely, neat, cross design I worked it only one way,
but had to fight with sewing the piece together as the finished piece
Discarded attempts as I got to grips with how different designs
and techniques work-up.