First establish your swingplane by swinging back and forth continuously with a short swing that grows longer and longer until it is full. Momentum will carry your club almost exactly on plane, not upright or flat. You could be inside out or outside in depending on how you make the backward swing or on whether you loop, but it will be the right plane for that swing and it will be an otherwise good backswing. This exercise also helps you establish the top of the backswing, and you can then build a backswing that goes to that top.

To build a backswing that goes to that top position, do the above exercise and then transition into a new movement that takes you from the top position down to the address position and back up to the top, down to address, etc. If you lose the top position, find it again and then repeat the backswing exercise.

This gives you the possibility to have a new correct top position and a backswing to take you there--almost a complete swing.