wow, but do you have to cut the tube at an angle or do you bend the tube inwards and shape it?
There are at least a couple of ways to achieve this look. One is to use a belt sander at a shallow angle to the end of the seat stay, then brass-braze a flat steel plate on to the sanded surface. By brass-brazing the cap, it won't come apart when the stay is silver-brazed to the seat lug.
The second way is to braze a manufactured plug to the top of the seat stay. The plugs are already finished with the shape designed in, but have the disadvantage of weighing more than the flat steel plate cap. Cinelli makes plugs of various shapes for different sized seat tubing.
For light duty frames you can use shot-in stays where the upper end is integrated with the seatpost clamp.
The classic method is good for light-med touring. There was a neat weight saving trick (shown by Peter White if I recall) using a reversed section of tubing (concave-out) cut form a scalloped end rather than a heavy investment cast plug. This example does it without the scalloped shape.
Heavy duty frames often extend the area of braze by using wrap-around plugs.