With describe, don't forget the general trend (i.e. graph is increasing from 0 to 20 minutes where it plateaus and declines rapidly after 25 minutes).
For explain, you are using your background biology knowledge so for example as temperature increases kinetic energy of molecules increases, so there are more enzyme-substrate complexes etc. and after 25 minutes are denatured.
Evaluate usually says "using information from the graph" and sometimes has a conclusion, so as the above person said "correlation does not imply causation". I tend to always squeeze in values from the graph since AQA have oddly specific marking criteria sometimes.
The best thing to do is have a look at some past paper graph questions, answer them and check what the mark scheme wants since it occasionally varies. Good luck!