Knitting stitch patterns are a lot of fun to use in projects, but you can't always find the project you want to make with the stitch pattern you want to use. If you're wanting to knit something simple like a scarf, shawl or blanket with a particular stitch pattern, it's really easy to design your own pattern.
Getting Your Gauge:
As with so many things in knitting, being able to design your own simple knitting patterns with the stitch pattern of your choice begins with gauge.
Choose the stitch pattern you want to use and the yarn you think you'd like to knit your project with, along with a pair of knitting needles of the size the ball band on the yarn suggests.
Knit a gauge swatch measuring about 5 inches square, or whatever size needed to fit in full pattern repeats but that will make a swatch larger than 4 inches.
Pay attention to the instructions for your stitch pattern so you cast on the proper multiple of stitches. For example, if you wanted to knit a scarf in Herringbone, you'd need to cast on a multiple of 7 stitches plus 1 (so, 15, 22, 29, etc.) for the pattern to work out right. It's always a good idea to work at least two full repeats (widthwise and lengthwise) of a stitch pattern so you can get a better idea if you like it.
It's a good idea to block your swatch before measuring it, treating it the same way you'll treat the finished knit item. Measure how many stitches are worked in 4 inches, then divide that number by 4 to determine your number of stitches per inch.
Making Your Pattern:
Once you know how many stitches you need for each inch of knitting, you need to figure out how many inches in width you need. For example, if you're knitting a scarf, you might want as few as 4 or as many as 10 inches in width, depending on the style of scarf.
For a shawl, you might want a project that's 2 or 3 feet wide, and a blanket could range from 36 inches square for a baby to as big as a king-sized bed (though you wouldn't want to knit that all in one piece!).
Once you know the width in inches you're looking for, multiply your number of stitches per inch by that width number. So if you were knitting a 6-inch wide scarf, for instance, and you got 5 stitches per inch, your number would be 30.
That's not necessarily the right number of stitches to cast on. Look again at the pattern stitch multiple requirement. Using Herringbone as our example again, we need a multiple of 7 plus 1. 30 is not a multiple of 7 plus 1, but 29 is, so we can cast on 29 stitches, work in the stitch pattern as long as we like or until we almost run out of yarn, then bind off and we've got our scarf.