I created this system using the 123D Design app for iPad, which is free. Follow these instructions to design it yourself!
Step 1: Quick Tips - README
1) When you want to select an object to manipulate with a tool, select the tool first
2) When you select an object, make sure you don't have any others selected already! Tap a couple times on an empty space.
3) When you manipulate your perspective on an object, use slow movements so it doesn't fly all over the screen; you will get super disoriented.
4) When you insert objects or duplicate them, space them apart so you don't accidentally tap on the wrong object. This will make it so you don't constantly have to deselect objects.
5) When you use the snap tool to make two objects come together, select the object you want to move first then the object you want to remain stationary.
6) When you tap an arrow to move an object, a positive value will move the object in the direction the arrow is pointing, a negative (-) value will move it in the opposite direction that the arrow is pointing.
7) To quickly orient the camera to the object, tap the "home" button
8) Keep your extra cylinders in one place spaced away from the box to keep a point of reference. This will help you know how you are oriented. In my pictures, my extra cylinders are to the right of the box. It would be good for you to do the same, it would make the pictures that much more helpful to you.
Remember, if you make a mistake, just hit the undo button near the bottom left. If you want to undo an undo O.o, hit the redo button near the bottom right.