Well It's pretty simple
- for your first layer you should go to layers then add an alpha layer to it, scale up the eraser and erase all the white so it's just alpha.
-Go to layers and create a new layer, if this is going to be the background I would suggest adding a background color.
- I'm not exactly sure, but I think all new layers appear on top, but they are selected when you first create them. So go to layers, then go to stack, then go to move to bottom.
- Now you have 2 layers, one completely transparent and one with a background. Select the top layer and draw whatever your subject is.
- When your done select the bottom layer and do your background, it may be easier to temporarily move it to the top so the main image doesn't get in the way.
Hope this helps