First click onion skin, second set the marker to two frames so that you can select the second frame. When you have the second frame selected, you will see a transparent element in my case its a stickfigure. Everytime you draw with a brush or other tool you can draw over the previous frame (The previous frame is now like a template for you to draw over). When done with the second frame just make a extra empty keyframe for the third keyframe by hitting F7 for Windows, for Mac its fn+F7 if you have a compact keyboard otherways its just F7.
Thanks for reading :)