Warm-up drawings are simple exercises but extremely important especially for beginners or for any artist which is not sketching on a regular basis. Warm-ups have a dual purpose, 1 - warming up the muscles of the hand and 2 - making the mind agile.

Give it a bit of context by practicing the following.
1 - Motion: Slow and fast. 2 - Pressure: Light and heavyweight. 3 - Size: Small and large sizes. 4 - Density: Dense or rare. 5 - Repetition: both, motion and forms. 6 - Any combination from above.

Start to work on a larger scale by creating more complex objects using with a mix of other shapes and textures. Use regular and irregular shapes and place them symmetric and asymmetric.

Once you get the hang of that, add in some variations and explore various possibilities of textures and materials.