I don’t necessarily subscribe to the idea that you should block out your time weeks in advance with fixed hours for revising different subjects. It’s more fun to be flexible – as long as you aren’t consistently choosing subjects that you find easy.
I do, however, believe that you need a clear idea of how you will approach your study before you begin. You should know which topic you intend to look at (based on a study plan), what technique you will be using and what the ideal outcome is.
Sitting down to revise with no plan is likely to involve flicking idly through a textbook wishing that you were somewhere else. With no direction it’s easy to waste your time, impossible to know when you’ve finished and whether you actually learnt anything.