control signals to draw a circle

[irving2008]

Essentially you are right. Suggest you read

Bresenham's line algorithm - Wikipedia, the free encyclopedia

for the basic line-drawing approach. While this will get you started however don't underestimate the complexity of handling the real-world mechanical issues of, for example, acceleration & deceleration, unless you can accept very low drawing speeds.

Both steppers could, and probably should, be operated at the same time to maximise smoothness of the line.