When I try to start a custom single player game on one of the maps, it automatically sends me back to map selection screen. What am I doing wrong?

I select the map, click More Info, then click "Start a single-player game on this map", and it sends me back to map selection screen.

That's not how you should start a game.
You only select it, and then click the next button below.

^'it' means the map.

Afer you've clicked the "Start a game" link, you'll notice that you are on the map selection screen of the game creation page. If you look closely, you'll see the map you want is already selected. So, you can immediately click "Next" to go to the next stage of game creation.

Oh... Thank you. I thought the next button was for the next page of maps list :)

That's an understandable confusion. I'm going to work to make it clearer. Thanks for pointing that out.