Good job, especially considering that you were working with MS Ain't. As for where the deserts would be, think of it this way...

Where are your tectonic plates; the plateaus on which your continents sit? Once you know where those are, you can place your mountains. Mountain commonly occur along a point where two tectonic plates meet. Deserts typically occur on the leeward side of mountains. The reason for this is because as the air is forced up the windward side of the mountains, it condenses into rain. By the time it crests the top, much of what is left is dry air. That's why it rains a lot in Seattle.