I started painting murals by accident. A teacher misinterpreted my school visit flyer and said, "Oh, I see that you paint murals too." I said "Er...yes!" And now I've done over 50.
I really enjoy the freedom of painting freehand on a large scale and usually have even a large mural planned out on the wall in a couple of hours.
If I'm working in a school I often start a mural project with a drawing workshop after which the children produce their drawings or paintings. I'll then paint a background and incorporate as many of the children's images as possible.
Either I'll recreate them or help more able children to paint their own (And tidy up the edges afterwards!) Alternatively I can do the whole thing myself which is considerably quicker!
Occasionally, my murals have a 3D element. I generally enlist the help of the school caretaker to construct a wooden base then build on it using chicken wire, a staple gun and various bits and pieces before covering it with plaster of Paris bandages and then painting and varnishing afterwards.
It can be a bit of a messy process but great fun and again I'm happy to work with the children 'helping'.