If your products are covered in wet mud, allow the mud to dry completely. Do not attempt to wash away any wet or damp mud, as this can cause the stain to spread or become embedded in the fibres making it harder to get out.
When the mud is completely dry, brush away any excess mud. Be careful not to push the dirt back into the fabric.
Soak in cold water and rub some washing up liquid on to the dirty spots (I find washing up liquid gets mud out better than washing detergent). If there are really stubborn muddy spots, then use a brush (toothbrushes work well) to lightly scrub at the stain. Leave to soak for 15 minutes.
If it's not completely clean after soaking, then pop in the washing machine at 30 degrees. If the items are really dirty, then you can also use stain remover.
Once out of the washing machine, you can just hang them out to dry.
Harnesses and poo bags can go in the washing machine (make sure to remove the metal clip from your poo bag), but we would suggest only hand washing leads to prevent the clasps scratching.