What is the Hardest Part of Revision?

For me, when I was doing my A-levels, the hardest part of revision was actually knowing where to start! I don't think I was ever taught what effective revision looked like, or how to identify it!

On the other hand, for many of my students, it's memorising quotes. Some of them struggle to know which quotes to memorise in the first place. Other students know which quotes to learn, but have no clue how to go about it!

What about you?