The last rule you write will override an earlier rule for the same selector(s), so where you need one rule to override another, makes sure it comes later in the CSS.
Other than that, structure your CSS rules in a way which makes sense to you, which will make it easy for you to read and maintain your code at a later stage.
As will many things in CSS, there is no absolute right and wrong for all situations. As a general rule, you should avoid repeating yourself - keep your CSS as short and simple as you can - but don’t do it at the expense of clarity or ease of reading.