Edit: The following method is OUTDATED because the PTD servers shut down
To play a Flash game in 2021, you need two things:
2) A Flash Player projector, which you can download here. After that, simply run the file through the projector. This works for all downloadable Flash games and content. If you're having trouble getting the Flash projector to work, download the game from this Google Drive link. These files run without the projector.
Consider joining my Discord. It's the most active PTD community where you can learn to modify the game or get general gameplay advice.