A Quick Lesson in How to Make a Swatch
Now that I've convinced you about the importance of swatching, let's talk about how to properly make a swatch. If you want your swatch to be accurate and relevant to your project, there are two factors that must not be overlooked: stitch pattern, and size.
The project pattern should tell you what stitch pattern to work your swatch in [e.g., GAUGE: 9 sts = 4" (10 cm) in St st]. Many times this will be stockinette stitch, but do be sure to use the stitch pattern called for if it differs. Lace and cable patterns will often have a quite different gauge than stockinette worked with the same yarn and needles. And remember, you're not just making a swatch for gauge: you want to see if the beautiful lace pattern you’ve chosen shows up when you work it with that deep purple yarn you fell in love with. Better to find out now than after you’ve completed an entire sweater back!
The size of your swatch is also very important. The standard these days is for pattern writers to tell you how many stitches and rows should equal a 4" (or 10 cm) square. This does not mean that you should cast on that many stitches and work that many rows and end up with a 4" square! Edge stitches and cast on/off edges will affect the size of your swatch. You should plan for a few rows and stitches of garter stitch border –- I usually cast on an extra 3-5 stitches total (3 for a larger gauge, 5 for a finer gauge) and work 4 rows of garter stitch at the top and bottom. (You will want to do this even if your swatch is in garter stitch, to allow for the edge stitches.)
Finally, you absolutely must remove your knitting from the needles before measuring your gauge. Don't stretch your work when measuring (even if it’s ribbing), and be sure to count fractional stitches. A quarter of a stitch doesn’t seem like much, but over several inches it’s going to add up.
Now that you've properly swatched, measured, and seen and felt how your finished fabric is going to look, you're ready to cast on your project. Good luck, and enjoy knowing that you've taken steps to ensure that your project is going to be a success!