A gauge is very simple to use. If the pattern calls for a 1 by 1 inch stitch gauge and says they have 4 stitches per inch, you make a gauge and measure it to see if you get the same amount of stitches per inch.
If you get MORE than 4 stitches you would want to go DOWN one hook size
If you get LESS than 4 stitches you would want to go UP one hook size
Then make another gauge and re-measure to see if it’s a better size hook for you to use for your project.
For a demonstration and more detailed explanation See Video: