Small manageable chunks of revision but starting now- she will have done hours and hours by summer. Our school have gone for an hour a day - some kids will obviously do more but we have a mixed cohort and this seemed achievable.
Absolutely agree that with a motivated child you can be pretty confident of a bounce up of a grade over the last 6 months so it is definitely doable.
Make sure she starts by making a plan of what she actually needs to do- it will also help to show her there is lots she already knows . This is something you can really help with- get her revision guide and mark up the pages she needs to learn.
I would see if she can find an hour breakdown that works for her so she feels confident what she is doing is moving her forward.
Something like- choose a section to revise
Work on understanding and memorising content watch a video / make notes or flash cards / mind maps whatever she likes to do
Finally- test herself- on simple content using eg flash cards or Seneca etc and then most importantly with past paper questions.
It’s this last bit kids are most reluctant to do as unlike making pretty notes it is painful and hard going- but it is the part that embeds memory.
Lots of marks are also exam technique rather than recall - to be honest probably a grade’s worth- so in science papers I can usually find 10 marks across a paper where a child has described when they should have explained, or forgotten to put an answer to the correct number of significant figures as asked etc. Practice past paper questions are again invaluable- you can find them on exam board websites and her teachers may have access to sites like exampro where it is possible to generate links to huge numbers of questions and answers very easily.
As she is revising she needs to make a note of concepts she is finding she struggles with and then ask her teacher. A short conversation can clear up misconceptions really effectively. I hope it all goes well for her.