Simple question:

If you make a custom map, where do you save it so you can play it on voobly with everyone else? And if you download a custom map, same thing?

I hope someone here can help me

If you mean like a scenario (.scx) then it belongs into the /scenario folder in your AoM installation folder. You can then play it single player through Menu->Single Player->Load Scenario or in multiplayer through hosting a room and changing Game Type under the map on the right to Scenario (other players will automatically download from you afterwards).

If you mean a random map script (so like a standard map like Marsh or Oasis), then the two files (.xml and .xs) belong to the /rm2 folder in your AoM installation folder. They will then be selectable in both single and multiplayer through the standard map selection process (other players in multiplayer will yet again automatically download).

Hope this helps