Make room to measure inside
Use the yarn, needle or hook, and stitch pattern intended for your project. Make the sample larger than the area you will measure, so the cast-on, cast-off, and side edges stay outside your count. For a 10 cm measuring area, a sample at least 15 cm wide and high gives you room around it; large stitch repeats may need more.
Prepare it like the finished piece
Follow your pattern and yarn-care directions for washing, drying, or blocking. Lay the swatch flat and measure without stretching it. If the project is worked in the round, use a swatching method that represents that construction.
For example, count the stitches across an interior 10 cm width and the rows up an interior 10 cm height. Those counts give your stitch and row gauge. If they differ from the pattern, change the needle or hook size and check another sample.