I've just found this post written by Michael Tipper on www.happychild.org.uk that explains clearly how to do that (and this is the way I do it!)
"First of all I never tell the kids that we are "Mind Mapping" or doing anything special. I tell them that we are going to learn about, for example, a farm.
I then ask them what sort of things do we see on a farm. I either suggest or try to encourage them to come up with generic words like Animals, Buildings, Crops, People, Machinery etc. These form the Key Images on the main branches because I will either ask them to draw an animal or a building etc or once again get them to cut pictures out from Magazines.
Then having captured the main branches I will go deeper into one of the topic areas for example I will ask what animals they may find on a farm and again sub branches for sheep, cows, pigs, hens etc develop and once again the children will generate these branches themselves. Now my description of this process is somewhat linear because as you are no doubt aware, young children will just tell you everything that they can think of without following my adult-orientated logical approach. I describe it in this way for ease of explanation but essentially what happens is that the mind map will grow and it will consist entirely of pictures structured in Mind Map form. In fact capturing the information this way is a great way of harnessing children's creativity and spontaneity. A more linear, topic by topic approach may stifle a child's natural desire to blurt out the first thing that comes to mind. Another good thing about doing it this way is that the children can work together in teams, one doing the "Animal" branch, another doing the "Machinery" branch etc.
If this approach is adopted when covering any topic, it will just seem natural to the children that it is the most sensible and fun way to capture information. Then if you ask the children to tell you all about the farm (or whatever the subject is) they will "see" the Mind Map, particularly the bits that they were responsible for, and will give a very comprehensive and structured account of a farm. If you are dealing with very young children who are just learning to read, you could label the Mind Map Images with large lettered words to help them recognise the words from the pictures.
For parents this is a great way of bonding with your child and exploring a subject together. For teachers it is a simple way of engaging young children in an activity that will help them learn and process information about a topic".
This three minutes clip, provided by MacGrercy Consultants (www.macgrercy.com) shows you the basics of how to make a mind map.
The intention is to show you the main points to get you started.
Try it and let me know how you get on with it!
More about Mind Mapping on this blog: