Gauge

Dear users and authors,
why do you need a gauge, and how do you make one?
Thanks for your helpful answers.

A gauge swatch is something most people would rather skip and just jump right into their actual knitting or crochet project. I used to feel that way, too. This might not necessarily be a problem for amigurumi, scarves, and other items that don’t have to fit perfectly. But when it comes to clothing, it’s actually essential. Everyone knits or crochets a little differently—some more tightly, some more loosely. If your own knitting or crocheting style doesn’t match the author’s, the measurements might end up being off and the garment won’t fit. In the worst case, you’ll have to unravel everything and start over. It’s definitely less work to do a small gauge swatch beforehand!
A gauge swatch is usually specified as 10 x 10 cm. For the gauge swatch, I recommend knitting a few more stitches and rows than indicated in the gauge. Then measure and count how many stitches and rows you needed for a 10 x 10 cm square. If there are fewer stitches and rows than specified in the gauge, you’ve knit more loosely—so use smaller needles or a smaller hook. If there are more stitches and rows, you’ve knit more tightly—so use larger needles or a larger hook!
It’s also helpful to cut out a template from cardboard: simply cut out a 10 x 10 cm square, place the “frame” on your work, and then count the stitches and rows within the square. So definitely give the gauge swatch a thumbs-up!!!

Here’s another great and very detailed description—thumbs up.

Some yarns stretch out after washing. Merino is one of the main ones. With these types of yarns, it’s recommended to wash and dry the gauge swatch the same way you would the finished garment.

Measure the gauge swatch before and after washing. If it comes out slightly larger after washing, then the finished garment can safely be a little too small once completed. If it comes out smaller after washing, then the garment should be slightly too large once completed.