Intarsia is a knitting colorwork technique that involves knitting with blocks of color. They can be in any shape or design you like, but the key is that when you change colors, you don't strand the colors you're not working with across the back as is done in stranded knitting (also known as Fair Isle).
Instead, each color of yarn is its own unit, with the strands being twisted together when you change colors to connect the pieces.
It's an easy technique, but it does require a bit of forethought because you need a different strand of each color to work within your design. So, for example, this tutorial uses a two-color block of 20 stitches and 20 rows, with a 10-stitch, 10-row block of the second color in the center of the first.
To make that work you'll need one strand of your second color and two strands of the first—one for each side of the square. For a small swatch like this, you can just cut long strands a couple of yards each, but for a bigger project, you might need actual balls of yarn to work with.
To set up your intarsia, knit any rows that don't include the color design. In this case, cast on 20 stitches and worked 5 rows, beginning and ending with a purl row.
Intarsia on the Knit Side
Now you're ready to begin working the intarsia pattern. In the case of the swatch shown, that involves knitting 5 stitches in red, 10 in beige and 5 again in red. As mentioned earlier that means we need three strands of yarn, one for the first part of the red stitches, one for the beige stitches and one for the remaining red stitches.
To begin intarsia on the knit side, work the first stitches in your background color, pick up the second color and knit the next stitches with it, then start a new strand of the background yarn on the opposite side. If you're following a chart, this will work in the same way.
Starting a new color of yarn is similar to joining a new ball of yarn at the edge of a piece of knitting, as you might have done when knitting stripes.
Remember to leave a few inches of tail each time you start a new yarn to weave in later and don't cut any threads you've been working with; you'll need them again on the next row.
Purling in Intarsia
On the purl side of the work, the process of knitting intarsia is pretty much the same as it was on the knit side. Now that we are no longer on the first row, though, it's possible to twist the yarns as we change colors to ensure that the different pieces of knitting stay together as we work.
To do this, pick the yarn you're about to start working with up from underneath the yarn you've just finished working with. This will twist the yarns and join the separate blocks of knitting together.
If you didn't do this at all, you'd have three separate pieces of knitting on your needle. If you don't do it consistently (on both the knit and purl sides), you'll have holes in your knitting.
Continue in this manner across the row, following your chart or, if you want to knit this practice swatch, continuing to work with 5 stitches in red, 10 in beige and 5 in red (or colors of your choice).
Now that you know how to work intarsia on the knit side and the purl side, keep doing what you're doing, following the chart you're working or whatever design is striking your fancy.
Remember that the threads need to be twisted each time you change colors, and after a few rows you'll probably want to straighten out your threads a bit, so you don't make too big of a jumbled mess.
To finish the sample swatch as shown, work the established center pattern for 10 rows, then work 5 rows in red (you can cut your beige and the other red yarn at this point) and bind off.