Do not wash the canvas shoes directly with water!
It’s better to wash the front part and the bottom with a used toothbrush:
Take a dry towel with a fingertip and take a little bit of water.
Then apply some toothpaste and gently rub the front and the edge of the shoe to the heel.
Note that the heel part should be especially careful and gentle.
Reclean the area with a wet cloth and it will be very clean!