CSS Organisation + HTML naming


what is the best way to organise your CSS stylesheet. I tried to finish the Fotomatic-Project, but find it really difficult, to understand the organisation.
Which elements will be first?
How/when using separations and how to name them?
Is there a common way or a “best” way and if yes, how is it done?

Thank you

Hi I organize my CSS by putting the * rule set first the the :root rule set.after that it’s the most used elements in the HTML.