I have been "grinding" coffee beans and making fresh coffee for years, but never really wondered if I was doing it right until now. Apparently I wasn't. I use a french press so need coarse ground coffee, but my coffee grinder is an electric one that only produces fine ground coffee. I was also only using about half the amount of coffee that I should have been for my 350ml mug. After buying some pre-ground coarse coffee and using about 20g with 300ml of water I could tell the difference. I had assumed that since I was using freshly ground beans then I couldn't really go wrong even if it wasn't perfect, but now I am glad I finally checked what I should be doing.
I found a handy website called Honest Coffee Guide which helps calculate the coffee to water ratio you need.