Every person (both children and adults) have unique ways of learning and studying that work best for them. The first step is to find out whether your child is more of a visual learner, kinaesthetic learner, auditory learner, etc.
Visual learners learn best through diagrams, so it may be useful to draw mindmaps and charts.
Kinaesthetic learners learn through movement, so writing down information helps to commit it to memory.
Auditory learners learn through sound, so recording things on the phone that need to be remembered and replaying that recording can also be useful