I do it in CAD. just regular CAD. Say one has a seatube of 1.125" Top tube, and Head tune. Both the verticals are 73 degrees say. I have different tube shapes pre-drawn, and I just drag them in and rotate them into position. But some people just have some rulers that are tube sized, or use drawing instruments. One the tubes are drawn in their actual position, you can see the points of overlap. So take the top of the tube say it is 560 mm long with the cuts. you cut one end to 73 degrees, now go back along the top 560, cut the second miter. The only odd bit is that you are measuring to the bottom of the notch. The actual piece will be longer overall, but how much depends on the thickness of the intersecting tubes, and the angles, etc... But it's easy, you just take the measurements off a drawing, and cut to the bottom of the notch,

It really helps to get past the BS stage as soon as possible, and measure your parts for your drawing from actual purchased parts. Then just start drawing, it will come together, and your questions will be answered from the plan. By get by the BS I mean that it helps to have the actuall BB, they aren't all the same size, the drops, real wheels that will be used. It isn't absolutely essential, but a lot of problems are easily solved with the real numbers.