Not actually a sketch; this is a "color" / spacing / coordination drill I just devised.

You start with a simple outline, like a circle, ellipse, box, or regular polygon.

Then you draw lines, adding an array of at least two similarly-dimensioned primitive objects. I recommend drawing the entire object without lifting the stylus.

Why at least two? Because the aim is to create spaces that are (or at least feel like they are) the same size, so you need a minimum of two spaces to compare.

After drawing each shape, check its spacing on each end, and its spacing vs the previous array element

Then you repeat (placing an array of objects into one or more of the shapes you just drew, and so on ..)

Things to keep in mind:

Avoid crossing lines. Try to make your lines stop exactly where they started, and strictly avoid colliding with the shape you are placing or a neighbouring array element.

Fill with simple shapes. All the shapes depicted here are essentially distorted boxes or ellipses.

Each 'array' should ideally feel like it 'flows' in a uniform direction. If it doesn't, you have probably got the spacing and hence the angles wrong.

Avoid the reduction of spacing if the outer shape's surface feels like it's "turning away". This exercise is about consistent measurement, not about perspective.

Shapes are 'good enough' if the spacing checks out on all four sides. This drill is not about drawing perfect circles or straight lines, either.. or even about drawing "shapes of the same size"; it's about having accurate spaces.

Another drill: Pick a lightsource location and a horizon, translate a cube through 2d space (that is, keep its angle to the screen constant)

There's obvious variants possible involving translating through X,Z or Y,Z rather than X, and/or rotating, skewing, or scaling instead of translating. Also, starting with a cube at a different angle to the screen.