I made a sketchnote handout for my students about good habits to form when learning observational drawing. Observational drawing is the simple act of looking at a 3D object and transforming it into a 2D artwork showing it as accurately as possible.
The simple piece of advice is to never keep your eyes off the drawing or the subject being drawn for long.
The more complex advice is to use the first thing you draw as your “moment of truth,” by which everything else you draw can be measured. Basically, just draw one part of the drawing and if something is twice as tall as it in real life, make sure it is in your drawing. Don’t be lazy, check everything. Feel free to use a measuring stick if you are really precise, but that is a whole article in itself.