I highly-recommend using the template map.
It's not worth doing everything from scratch, it only gives you diminishing returns, if at all. Trust me, I've tried all day yesterday, only to realize that it was a big waste of time.
The map works perfectly fine when uploaded and tested online, it's just testing it offline that's a hassle (but there exist a workaround, see the comments).
Remember, this is a 2-player concept, so you have to test it online sooner or later. Trying to adjust things just to test it offline will only bring you so far.
Take the example map, work on the terrain and triggers for the objective, and then upload it so you can test it with other players.