One thing to remember is that obviously over the time leather of the sole gets dirty. To clean the sandals Nörd Skin you can follow this hack:
It is totally not recommended to put the sandals in the washing machine and the best thing is to do everything by hand. To eliminate the dirt from the leather sole you can use a wet cloth with some soap for leather. Or you could do this DIY trick using a face cleansing milk. Rub swiftly onto the leather. You should also remember that leather is a porous material so it will collect dirt and sweat, so to avoid bad smells, clean them often.