It's not very complicated, I promise.
For reasons that don't concern us at this juncture, I recently found myself in possession of a truly ridiculous amount of Knit Picks Wool of the Andes Sport. Eight extra skeins - eight!
Naturally, none of the projects I want to knit call for this yarn. And none of the projects that people have used this yarn for on Ravelry are exciting me. Being a stubborn sort of person, I want to do the project that I want, in the yarn that I want, end of story. Luckily, it's not too hard.
Step 1: Get as close as you can
The best way to compare yarns is by yardage. Ignore terms like "worsted" or "fingering." There's so much variation. Instead, write down the number of yards per ounce or grams. In the case of WOTA Sport, each skein is 137 yards per 50 gram ball.
The closer you can get to those two numbers, the better. Ideally, within 10 yards. For example, Cascade 220 is 220 yards per 100 gram ball. The equivalent of 2 balls of WOTA Sport, which would be 274 yards per 100 grams. At 54 yards off, that's not a great match, but it's within the realm of reason.
Step 2: Swatch
Ignore what it says on the label. The label is always wrong. Knit up a swatch and see how many stitches you are getting per inch, with a fabric that you like. Try to avoid the urge to fudge the numbers - the more realistic you are at this stage, the better your end results will be.
Step 3: Start cranking out the numbers
What you're going to do is convert all of the numbers of the pattern. The best way to do this (I have found) is to actually write on the pattern. Scratch out all of the numbers and write in your own.
Let's say my WOTA Sport is working up at 7 stitches per inch. But I want to use a pattern that calls for Cascade 220 at 5 stitches/inch.
The pattern has you start by casting on 50 stitches. In Cascade 220, that's 10 inches. (The number of stitches divided by the number of stitches per inch, or 50 / 5).
Now convert the numbers back: 10 inches of fabric in WOTA Sport would be 70 stitches. (The number of inches multiplied by the number of stitches per inch, or 10 x 7).
The fewer of these calculations you have to do, the better - and the more precise your end result is likely to be. That's why I would recommend a yarn substitution like this for something simple like a scarf or a basic sweater, but not for something intricate like a glove (with the fingers and the shaping and all).