I think that what we're seeing here is a sphere with three circles that cut it in two equal halves. The best way to approach this is to start with a cube, and then cut it in two, in three possible ways, and from there construct the circles (I will cook up an example today). Although this a good exercise, in practice most artists will simply eyeball this, and another good exercise is to tighten rubberbands around a sphere, like a tennis ball, and draw this as a model.
Quick drawing messy I am at work 😃 had to hurry. Basically what you need is really good perspective knowledge, to train your self to think in 3d. I would start with box like the drawing understand 2d practice ellipses then box then try sphere, hope it helps
Just a simple student but I think is just a sphere with an indicator of the 3 axis: Longitudinal (top to bot), transversal (left to right) and anteroposterior (front to back). Is like a beach ball with different colors. Artist has to choose a point where 2 of the axis meet (long.+trans., trans+antero, or antero+long.), then draw the other one missing. In a sphere you cant meet the 3 at the same time. Maybe you can help yourself getting a polyspan sphere (they are so cheap) and putting around rubber bands, ponytail holders(?) Sorry, not native. I hope this helped, greetings!