While I understand the idea of tracing the plane line, and it's easy to get on a horizontal plane, my sense of whether I'm doing it right on an inclined plane has always been a little hazy. Here's something that's helped me.
Take a mirror and set it up so that it is angled about 20 to 25 degrees from the ground. By looking into the mirror from your setup position, you can now effectively see the plane (as a single straight line on the mirror). A smaller angle (approx 20 degreees) will work for the turned shoulder plane, while a larger angle will give you the elbow plane (aprox 25 degrees). If you can draw a straight line on your mirror, all the better.
Using a short club so you don't smash the mirror and gain seven years worth of double bogeys and lip outs you can now check.
- that you trace the plane line at takeaway and through impact
- that your right forearm gets on plane through impact
- that you finish swivel on plane
- that your right shoulder finds the plane at the top (tsp only)
- that your rght shoulder traces down the plane
- that you're not laid off or across the line at the top
- if your shifting (e.g. double shift) that you refind your original plane
- that you maintain a steady head etc.
And unlike with checking in a mirror behind you, you don't have to look up to check all this.
Setting up the mirror sounds complex, but it's not too hard if you just experiment with it. By adopting a smaller angle you can also practice tracing the plane line for putting this way.