Oh swatches. Knitters do seem to love to hate them. I love to swatch - I must be weird.
What I tend to do is, while I'm knitting a current project and I have that moment where I'm either bored or a little frustrated with what I'm doing. I get a ball of yarn for my next project and make a swatch. I get to play with my new yarn and also, make sure that what I'm going to do next is going to fit.
A swatch doesn't take long to make - probably half an hour at most and I'd rather do that than have to reknit the garment. Just cus I'm lovely and a little bit bossy, I'm going to tell you how I make my swatches. Little teeny swatches aren't the most helpful of things, because they'll lie to you. A good hearty sized swatch will pay you back that bit of extra effort.
Let's say the pattern states that the gauge, that it's worked to is 22 stitches by 30 rows in stocking stitch across 4" [10cm] on 4.00 mm needles in a DK weight yarn.
What I'd tend to do is cast on about double the stitches, so something like 44 stitches (it doesn't need to be exact - I'd probably round down to 40).
Knit 3 rows.
For the main part, every row should start and end with 2 knit stitches, to give a helpful garter stitch border (this makes it lie flat, so much easier to measure it). So my first wrong side row would be, k2, purl to last 2 stitches, k2.
Here's a brilliant tip that I picked up from Ysolda Teague's Little Red in the City. By using yos, k2tog and p stitches you can mark what size needle you are using. I've found this really useful, for when I've done several swatches on different sizes or if months/years down the line I use the same yarn again, I don't have to do another swatch because all the information is there. So for this example ...
On my next row, I'd knit a few stitches past the border then, (yo, k2tog) four times, knit to the end. If the needle size was 3.75mm, then I would change it to (yo, k2tog) three times, then, (k1, p1) three times. Each yo represents a full mm and each purl bump is 0.25mm. Think this sounds far more complicated than it is to knit.
Knit the swatch in stocking stitch with the garter stitch border until the piece measures around about 6" (I can be a little lazy sometimes with this and only get to about 4/5"). Knit 3 rows in garter stitch and cast off.
I always treat my swatch as I would the finished garment, which means that generally I'll wash and block the swatch. It is useful to measure your swatch before blocking, so you can check your knitting isn't going off gauge.
Use a ruler (not a tape measure - it'll be more accurate), and count how many rows and stitches you have in a 4" square. It can help to mark with pins, where you start and finish counting. Remember to count 1/4 and 1/2 stitches as this does make a difference to the size of the garment.
Generally, I'll see how far off the swatch is and alter the pattern to fit, but if you're not comfortable with that then reswatch on a different size needle to get gauge. Go up a needle size if you have more stitches in 4" or down a needle size if you have less stitches in 4".
Sometimes, if the pattern is complicated, I'll do another swatch in the stitch pattern too. It does sometimes depend on what information the designer has given you for the swatches. I always think it's a good pattern if they give you a swatch for stocking stitch and for any stitch pattern (if it's different).
Swatching can be fun (really) and take away the frustration of spending months on a project, to find it doesn't fit.
That's the end of the teacher bit. Here's a sneak peak for a upcoming pattern: