Swatching in pattern
After swatching in pattern you may discover that your chosen yarn/ pattern pairing do not go together at all; the stitch definition may get lost; the colours may over power the lace; or the pattern that looked delightful as a chart might be a mess when knit up. On the other hand it may get you even more excited to cast on and start your project.
How many stitches to cast on
Stitch dictionaries tend to include helpful information like 'cast on a multiple of 5 + 2.' This means that in order to follow the directions as given you should cast on any multiple of 5 stitches, plus 2 (7, 12, 17, 22...).
The additional stitches are used to create a balanced pattern. This number does not include any edge stitches and you will probably want to add 2 or 3 stitches of garter stitch at each side. So for a pattern with a multiple of 5 + 2 I might cast on 38 sts for my swatch and work 3 stitches in garter at each end.
Extracting a stitch pattern from a project pattern
Patterns for shawls or sweaters often incorporate the instructions for a stitch pattern into the general directions. Look for the directions for a full repeat and carefully count the number of stitches in the repeat. If the pattern is worked flat or in a panel it may be obvious how many additional stitches are needed to work a balanced pattern, but it really doesn't matter if your swatch is symmetrical, so don't worry too much about this! For an example check out this post on extracting a swatching pattern from the Sherilyn shawl pattern.
Swatching in the round
Often stitch patterns will have different directions for working flat vs in the round. Whether you're knitting flat or in the round can also affect gauge so it's important to work the swatch the way the majority of the project will be worked. The fact that you won't need to convert the stitch pattern directions is just a bonus!
Measure using pattern repeats
Once you've finished and blocked your swatches you'll need to measure them to ensure you've got the correct gauge. You may find that counting individual stitches or rows in stitch patterns more difficult to count, but since you can figure out how many stitches/rows are in a pattern repeat it can be faster to count pattern repeats. In the example below there are 4 rows per pattern repeat so 10 repeats x 4 row = 40 rows for 4 inches.
Measuring motif or cable width
Sometimes it's even easier than that and the motif or cable width is given, add some stitches to either side of the motif so your swatch is big enough to spread out and then measure the motif.
You can find even more information on swatching on pages 18–29 of Little Red in the City.