Organisation is paramount. If your child tends to be disorganised in their work they may need your help in ensuring that they know where their revision materials and notes are. Always ask them to show you. Go through each subject in turn. Electronic material should be organised in folders.
Remove distractions whilst they are revising. Most importantly their phone. They should be working in short 20-30 minute bursts, so they can look at the phone during their break if they really need to.
Check that they have done a revision timetable. There are resources on this website that will help with this. It would be even better if you can make the revision timetable with them. Create a new timetable for each week. Include social time and activities. Put it in a prominent place in their study area. Help them keep to it.
Ask them about the revision techniques they are using. There are many excellent techniques described on this website.
Find out what they have the most difficulty with in a subject. Encourage them to review this work first and most often, as the tendency from many students is to review the work they can already do well. They should be using a system such as colour coding (red, amber, green) topics using the subject syllabuses.
Help them understand command terms. e.g the difference between describe and explain or analyse and evaluate. You should find command terms on the subject pages of this website, or on Google Classroom or you can do a web search for them for each subject.
Students learn better if they create something in their revision activity, such as midmaps or flashcards. Display these creations around the home. Talk about these creations and test them on them regularly.
Gamify revision, e.g. create a bingo game with flashcards.
Ensure that they are doing plenty of past paper practise. Even better if they do timed papers or selected questions with no notes to look at.
Ensure that they are getting plenty of sleep and exercise.