To be honest I never really paid attention to gauge when I crocheted, but back then I was just making a lot of afghans. With those you can kind of fudge a lot of things. But once I started venturing into other things like hats and scarfs I realized that gauge is really important.
Not everyone works up their projects with the same tightness so you might find that you crochet a lot tighter or looser then the designer. It’s pretty easy to figure that out. Almost every pattern that I have worked up (unless it really doesn’t matter about gauge) has some sort of way to see if you are working up the project the same way. I highly suggest making a swatch before EVERY project.
Some designers state the letter for the hook that they used while some use the mm size of the hook. I have noticed that between different brands of hooks the size could be a little off. I use both Susan Boyle and Bates hooks (even though I do prefer my Susan Bates ones) and the same letter hook has a different mm size to it. So, always check that before you start working.
It’s pretty easy to adjust your gauge if it doesn’t match what the designer says in the pattern. It you have more stitches in the length then what the designer has stated (ex: Designer says 10 sc in 4 inches and you have 12 sc in 4 inches) then you just need to move up to the next size hook. Start the swatch over again and see if that matches better.
If you work up a swatch your swatch has less stitches in the length that the designer stated (ex: Designer says 10 sc in 4 inches and you have 8 sc in 4 inches) then you just need to try moving down to the next smaller hook size and rework the swatch again.
Very rarely have I ever had to move more then one hook size either way.
Below is a picture of two hat bases, the right one was made with the size hook stated in the pattern without a gauge swatch…. the left was made with one hook size smaller. After I made the swatch I saw that I had less stitches then the pattern designer. So, you can see how important it is to make sure the gauge is right… the first hat would have been WAY tooo big! You could actually fit the left hat into the one on the right!