Observing, comparing, dividing...
To draw accurately, we must learn how to measure - and not with measuring instruments, but with the eye, observing, comparing, dividing, etc...
One of the best ways to acquire these skills is drawing from observation. We can see in the example opposite that the rough sketch of a castle cannot be done if we cannot "measure" the different elements that compose it. The easiest way is to start drawing a part - the middle line of a wall, for instance -, and to use it as a reference (how many divisions of this wall make up a window, how wide is it in comparison to its height, etc.).
Of course, at first it will be easier to take less complex objects that we can find around us, like a bottle and a glass, and draw them by comparing their proportions, their relations, etc.
Another interesting exercise is to simply draw lines and have fun dividing them into 2, 3, 4, or more. You just have to check if your lines is not too wrong and correct them as you go. It is also fun to draw a line and then redraw it in the same length, but further away or in another direction. These simple little exercises quickly give you a sense of proportion.
When you make a drawing like the one below, you can strive to draw precise perspective reference lines, but it is likely to be tedious and can harm the spontaneity of your drawing. Being able to detect distances from observation allows you to get your proportions right, so that making visual comparisons will become easier than precisely measuring every detail. Let's take an example from this drawing. Let's draw a line to mark the bust size of the standing man. To make sure that this size is consistent with him, simply draw the same line and reduce it a little. It is fast and efficient.
A method that is very often used - especially when drawing from observation - is to use a pencil as a guide. If you keep the subject always at the same distance from the eye, you can use the distance from your finger to the tip of the pencil as a reference for what you are observing. Personally, I find it more efficient to practice without any tools, but if you can effectively work better using this method, there is no reason not to use it.