There are two types of custom maps. Standard maps, and Bun Race maps.

Standard maps can be downloaded here: https://steamcommunity.com/workshop/browse/?appid=400910&requiredtags%5B%5D=Map+-+Standard

Bun Race maps can be downloaded here: https://steamcommunity.com/workshop/browse/?appid=400910&requiredtags%5B%5D=Map+-+Bun+Race

Subscribe to a workshop item (a map) for the map to appear in game.