Summarize:

Transfer the snake from the enclosure and into a small, plastic container with a lid. Make sure that the lid has small holes so that the snake can breathe, but that the holes aren't large enough for the snake to escape. If you cannot find a lid with holes, use an electric drill to create some. Remove the old substrate and discard it. Replace it with fresh, new paper towels. If the garter snake eats worms or fish, the substrate will need to be changed more frequently than if the snake is on a mouse diet. Remove feces and urea every week from the enclosure and add small amounts of new substrate to top it up if necessary. Completely remove the substrate every couple of months to replace it. Mix a few drops of dishwashing detergent into a bucket of warm water. Remove everything from the enclosure and use a cleaning rag to wipe down the enclosure walls and floor, the hides, the cage furniture, and the water bowl with the soapy water. Dry the enclosure fully using a clean rag before setting up the enclosure again and returning the snake. If you are using a paper towel substrate, in addition to cleaning the whole enclosure every few months, simply wipe down the glass where the paper towels are soiled when you change them. If the cage furniture cannot be cleaned, such as objects made from cardboard, replace it when you clean the enclosure. Cage furniture that is made from plastic or wood can be wiped down and dried.
Remove the snake before you clean the enclosure. Change the substrate once per week if you're using paper towels. Replace the substrate every few months if it's shavings, mulch, or bark. Clean the enclosure completely using soapy water every 2-3 months.