How do you clean the inside of this dog house?

Depending on the amount of grime buildup most household cleaners will work.

I have owned one of these for over 17 years and they are great. To my surprise when the temperature was zero outside the temperature inside with hay and a door flap on the door was very reasonable. I cleaned mine with my power sprayer. It only took 30 minutes to clean the inside and outside. It always looks brand-new. If you don’t have a power sprayer you can rent one. Or you can spray bleach and water solution all over it. You must take it apart to be safe when you turn it over. Then take a strong plastic brush and dip it into a warm bleach solution and scrub. Use gloves and protective eye wear. Good luck!!