2. Press Shift+Control+N to create a new layer. A new window should pop up. Press OK.

3. In your Layers box, you'll notice a new layer has been created. Now, go to your toolbox and click on the Rectangle Tool.

P.S. Make sure your foreground color is what you want it to be. If you want your transparent box to be grey and your foreground color is white, it won't work. Your foreground color has to be black.

4. Now, with the Rectangle Tool, draw a box that you want transparent at the place you want it to be. Don't worry if your box doesn't look transparent.

5. Go over to your Layers compartment again. You'll see something that looks like this:

Change your opacity to whatever you want. I changed mine to 74.

6. Now, all you have to do is save it and you're done! Mine turned out like this:

I know it looks horrible but hopefully yours will turn out much better. xD If you have any questions, just post!