Just like on paper. Do a lite sketch over sanded primed walls. Use watered down paint. Have fun.
This is a pretty cool tutorial: https://mrkate.com/2016/05/18/diy-watercolor-mural-wall/
I probably would not use water color as it is a more demanding medium. I would use acrylic paint. Draw out your design on graph paper. Lightly pencil in a graph on the space chosen. Pencil in the design to match the graph paper design and then paint away!
here is one of my favorite murals! https://www.thegoldhive.com/blog/2017/10/19/the-tricks-to-creating-a-perfect-wall-mural