Test knitting is an important part of the designing process. So what can you do to be a good test knitter?
Let's start with what test knitting is: basically, a designer has finished a pattern and wants to make sure that other people are able to follow the pattern and get the intended finished item.
Read the guidelines for the test knit and be sure to follow them
Many designers will have a list of things they want the test knitters to do, such as check gauge/finished measurements, make sure instructions are understandable, check yarn usage, check for errors. The designer also usually has a specific deadline in which you should complete the test knit. Make sure you are clear on what the designer wants from the test knit and that you can meet those requirements before you volunteer.
Be realistic about the deadline
Although some designers will be flexible on the deadline, don't assume this is the case. Many designers have a production schedule, and if test knitters are late in finishing a test, that can completely mess up the schedule. If the designer wants a beaded laceweight shawl finished in four weeks and it usually takes you four months to knit a beaded lace shawl, don't sign up for that test knit. If you know that you have a lot going on over the next four weeks and you won't be able to start the test knit until at least three weeks into a four-week test, don't sign up for that test knit.
Be realistic about how many things you can test knit at once
Along with the above, make sure you're not taking on too many test knits at once. Being a good test knitter means really paying attention to the pattern and your project, and if you have too many of that type of project going on at once, it can be mentally and physically exhausting and you might not be able to complete every test knit to the best of your ability (if at all).
Stay in communication with the designer
Life happens. Sometimes something unexpected comes up and you're not able to finish a test on time or at all. (For example, someone broke her arm during the time she was test knitting for me and thus couldn't finish the test knit. I hope nothing like that happens to you, but I do understand when it does.) As soon as you can, let the designer know what is going on so they can make back-up plans. Otherwise, give the designer frequent updates on your progress (I ask for weekly updates) so the designer knows you haven't fallen off the face of the earth. (Unfortunately, there are people who see test knitting as a way to get a pattern for free and sign up with no intentions of ever actually finishing the project as a test knit. Silence on your end makes the designer worry that you are one of those people.)
Don't skip the gauge swatch
I know. You don't like to knit gauge swatches, especially for accessories like shawls and cowls. But one of the important parts of the test knit is to make sure the designer's gauge is something that can be matched (whether or not that be with the listed needle size). If test knitters are unable to match the gauge no matter what needle size they use, the designer really needs to know that. So, a little tough love here. If you're not willing to work a gauge swatch, don't volunteer to test knit.
Follow the pattern as written
Most of the time when you're knitting and you want to make an adjustment to the pattern, it's totally okay. A pattern is just a starting point, after all. But when you're test knitting, it's not okay to make adjustments/changes to the pattern. The purpose of the test knit is to make sure the pattern works as written. So if you do something not in the pattern, you're not doing a true test knit. Once you've finished your test knit, you can make the pattern again and make all the changes/adjustments you'd like. But do it once just as stated. (The exception is any changes to be made because an error has been found. However, make sure to tell the designer and get confirmation that it is indeed an error before making the change.)
Provide the designer with feedback on the pattern. If you come across something that you think is an error, be sure to contact the designer right away. If you come across something that you feel isn't explained very well, be sure to point that out also. Your goal as a test knitter is to make sure the pattern as written will result in the intended item, so if you come across anything that might cause that not to happen, the designer needs to know. Make sure to focus on the actual instructions, not the design of the item itself. The exception would be if you come across a fit issue, especially if you're test knitting a garment.
Create a project page on Ravelry and take good pictures of your finished item
Besides checking to make sure a pattern is knitable, the test knitter's job has one more extremely valuable aspect: letting other knitters know that more people besides the designer has made this pattern and that it looks good in different types/colors of yarn. Think about when you are trying to decide on purchasing a pattern. Let's say there are two shawl patterns that have just been released that you're trying to decide between. One shawl has only one project attached to it: the sample from the designer. The other shawl has half a dozen projects already, and the shawl looks great in all of them. Which pattern are you more likely to buy? So, being completely honest, as a designer, having test knitters who create project pages with good pictures really helps a lot in helping a pattern sell. So if you're test knitting because you like this designer's patterns, I'm assuming that means you'd like to see this designer succeed and thus want to design more patterns in the future.
Signing up for a test knit
What if you've never done any test knitting before and you're worried a designer won't pick you if you try to volunteer for a test knit? If you've never test knit before, having a nice portfolio of your projects on Ravelry is a great way to show a designer that you might be a good test knitter. Another behind-the-scenes, honest bit of information here. If you're new to test knitting (either for that specific designer or just plain new), the designer is going to research you. That means looking at your projects (to check your skills and things like your picture-taking abilities) and checking your forum posting history (mostly to see if you've done test knits before and if you've completed them). So make sure those types of things are in good shape before you volunteer as a test knitter.
Finally, as I said at the beginning, test knitting is an important part of the designing process. Thank you so much to those of you who volunteer to test knit. It means the world to us designers that you're willing to use your valuable knitting time to work on a project under a deadline, with the potential for errors in the pattern, and without the ability to customize your project. Without your hard work, patterns wouldn't be as polished upon release. (Tech editing is also an important part of the process, but tech editing doesn't always catch everything.) I appreciate your work more than I can say. Thank you.