Put a ball at a spot in the circle, get a long rope, tie it to the goal posts and pull the middle of the rope to the ball. When the keeper is centered between the rope, he's on goal. Let the keeper see it from the shooter's perspective and from behind the goal. Move the ball and shift the keeper. Angles start with being on line and can be an abstract concept for keepers, young and old.
Depth is the second part of angles.
Going back to variables, how far a keeper plays up on the line of the shot will depend on his strengths and weaknesses in relation to where the ball is in the circle, as well as the keeper's own size, flexibility and reactions. With a simple shot from the top of the circle, a small keeper is going to have to play further out than a big keeper does to cover the same area. Effectively playing angles means building on your experiences and strengths. To find out what the best distance for a keeper to play off his line is requires patience, discipline and players willing to work with the keeper, especially with beginning and younger players. It does no good for a keeper to stand in goal for a hundred shots if they're coming from everywhere and going anywhere.