Look at the example provided here and remember the key ideas:
1. Use different colours for the arrows - a different theme should have a different colour (in the example you can see the light blue arrows are all to do with things that can be done in the home to reduce global warming, the green arrows are things that people can do as individuals and the black arrow is all about transport and climate change)
2. Have different size writing - make the more important information larger in size than the less important information
3. Use pictures and few words - only have the key facts and key words on your mind map. They should trigger memories and thoughts and help you remember the important information