After several tries I've realized that anything to do with 1PP triangle is very simple. You just draw it. Any kind you wish. As simple as that. Just change your Vanishing point (aka a triangle top corner).
It gets more complicated when it's a 2PP. And here, apparently, it's the best to draw a rectangle and then work from there.
You draw a rectangle, find its middle and draw a line to its upper side edge. And then you connect the three points, as shown.
It's not too complicated but I'm surprised that there is no tutorial on the subject. Should I write one?