It's fairly simple, actually, you can just use a flashlight that has a relatively focused beam. Swing it hip to hip, tracing a straight line on the ground.
As always, a good, balanced, arms hanging, setup, and neutral grip is key.
You can get any of the standard laser pointers, and just tape it to a club, one pointing up, one down (be careful not to shine in your eyes, very bad).
As long as you are tracing the line with both lights, going back and through, you are on plane.
Feel your 'center' (basically sternum) and from hip to hip, the light on the butt of the club stays relatively fixed there, while the other traces the line (or comes just inside).
Be aware of your hand/forearm rotation when doing this. There is some slight rotation, more on the downswing, but it stays in synch with your torso rotation (same tempo). Experiment with 'lots' of lead forearm rotation on the downswing, when you feel the hook, and the back of your left hand/clubface, back off and get your forearm and body rotation in synch. A metronome really, really, really helps with this.
Watch Payne Stewarts swing, his is a great example of the free flowing tracing of that line, and in synch/tempo arm rotation.
All the better to do this with a slightly heavy club, it will help you get the 'swing' of this.
The momentus strength trainer is great one, don't get one that is too heavy, but having the weight above and below the hands really helps you get the one peice takeaway feel, on plane. The standard momentus is great too, but doesn't quite help that takeaway feel as much.
Also, experiment with the feel of coming inside/out, and outside/in, especially with the heavy club. When you know the extremes, you can get the middle.
Let the club swing you, then feel like you swing it (your torso).
When you go back to your clubs at the range, imagine the target line, and imagine you are shining that light out of the end of the hosel. Straight back, very slightly inside, but the clubhead stays outside the hands, don't roll it in. And keep the face/back of left hand from too much rotation (experiment with extremes).
The clubface rotates slightly on the shaft, but the shaft stays on that line.
Hope that helps.