First, make sure it’s raining. Very wet weather is best.
Next, put on your raincoat; the sky-blue one that you nicked from The Cottage, the old kind that’s rubber over canvas.
Take a route that passes through long grass. It will be your scrub-brush.
Splash in puddles to rinse.
Don’t worry if your meander eventually takes you through more mud. There will be water enough to clean your boots again on the way back.
When you return home, evaluate them. If they’re not clean enough, repeat on the next rainy day.