Edit: I might be forgetting something, but it appears my MSPaint version no longer supports negative values in the skew transforms. Use photoshop or similar software to apply the transforms or stick with an older version of paint.
Edit 2: If you have a multiple monitor setup, you can use the following trick with OnTopReplica for greater precision in painting the land/water:
- Run OnTopReplica and select as window aoe2:DE
- Now rightclick again to select a region of the game window to put on your second monitor
- Open up a copy of the minimap you made in a new (mspaint) window
- Make the OnTopReplica close to full screen and using Ghost-it, overlay the second minimap mspaint window on top of the now close-to-fullscreen minimap region, so you have a close-to-fullscreen overview of what you’re doing ingame.