Easiest and most efficient: Use the Window/Workspace/Paint menu
This will give you a layout designed for using the paint tools, and the Layer Panel will be on the Right, the Comp panel on the left so you can see what your paint looks like in the comp while you work on a layer.
Another option is to open the Window menu and select Paint from the list of available panels.
I have edited my workspace selections in the toolbar and customized each workspace for my most common tasks because well-thought-out workspaces save time, and opening random panels clutters up the workspace.
You said you want to make the previous frame available. Are you talking about onion skinning? If you want to do that in After Effects and you want to do it efficiently the best 3rd party solution is Paint from AE Scripts. If you need to paint on 3D objects try Paint and Stick for a bit more cash. Paint costs about the same as a basic AE artist would charge for an hour of their time so it's pretty affordable.
If you want to fiddle with creating your own onion skin tool in After Effects you'll have to do a bunch of fiddling. The UI isn't going to be as responsive and it is not nearly as easy to work with.