I used this method for many years. It works well for heeling square gravers. I think it'll work for heeling 120's if you have the type that are full length pre-ground. If you have reshaped the front section of a square graver to 120, there might not be enough of a flat surface to stay true on the guide.
A sharpening fixture is always the best bet for repeatable accuracy, but this can get you in the game for very little investment.
For the guide I used a 4" Momax cobalt graver blank from Brownell's. Not sure if they are still available or not. Any similar square blank made of HSS should work. Just don't use a tool steel that's softer than the graver or it'll wear very quickly.
The distance from the graver point to the guide determines the amount of lift (heel angle). The closer to the guide the greater the lift, etc.
As in the top view, sweep the graver back and forth (left to right) across the whetstone, then flip to the opposite side of the graver to heel that side. The graver must be held so that its flat side rides against the guide to ensure a perfectly sharpened heel facet. Strive for symmetry as a larger heel on one side can cause the graver to favor one direction when cutting. Accuracy is the name of the game.
TIP: Slightly round and polish one of the corners of the guide to reduce drag with sharpening.