With the intarsia method of colour work, each area of colour is worked with a separate ball of yarn. As you work, the yarns are twisted where they meet so the blocks of colour are held together. Intarsia is best worked over stocking stitch although moss stitch and garter stitch are also effective.
You need separate balls of yarn for each colour used. Cut a length of yarn and then wrap it around the top two fingers on your hand, followed by the bottom two, in a figure-of-eight motion until you have a neat spool. Slip it off your fingers and wrap the yarn around the centre several times to secure.
Intarsia on a knit row:
Work to the point where you need to change colour and insert the right-hand needle into the next stitch knitwise.
Take the working yarn (the first colour) over the top of the second colour.
Drop the first colour and pick up the second colour. Make sure that the yarns remain twisted at the back and continue knitting.
On the right side of the work there should be a smooth join between the two colours.
Intarsia on a purl row:
Work to the point where you need to change colour. Insert the right-hand needle into the next stitch purlwise. Take the first colour over the top of the second colour.
Drop the first colour and pick up the second colour, ensuring that the yarns remain twisted and continue to purl.
This is how your work will look on the wrong side with an interlocking, vertical join between the colours.