Understanding proportion is fundamental to drawing. It is what makes the subject you're drawing actually look like an object.
Proportion is about how each element relates to the whole. Drawing is a test of patience. You have to constantly see where each point is in relation to other points. Always ask questions like "Is this point higher or lower, more to the left or to the right relative to this other point?", "Is this line longer or shorter?", "Closer to the point or to the midpoint?", etc.
For this tutorial, I've chosen to draw a simple chair by looking at a reference photo. Just to make things simple, I've rendered the chair as just a stick illustration.
Next, we go down the back of the chair to draw line BD. Point D is actually somewhere between A and B even though it's quite far below. To locate D precisely, use an imaginary guideline that extends down from AB. You will notice that the intersection of the imaginary line on line AB is much closer to B. Also, BD is going to be longer than AB. With D's location in mind, draw BD.
Now to finish the back of the chair. Using a horizontal imaginary guideline, you'll find that C is above D. With a vertical guideline, you'll find C to the left of A. With C roughly in mind, draw CD, and then AC.
When you extend a vertical imaginary line above F, you see that the intersection is somewhere closer to C on the CD line, and very close to A on the AB line. To determine how low to place F, using horizontal imaginary lines from C and from D, you can see that D is the vertical halfway point. Draw DF.
Keep in mind that H is lower than G but still quite close, then draw DH.
JFor EI, you note that I is lower than H, and it's at the midpoint FJmid and J.
To recap, to precisely place a point, you just have to ask yourself where that point is in relation to other points. Whether it's higher, lower, nearer, further, to the left or right, to the far end point, or to any midpoint.
When you're drawing outdoors, you can get a guideline reference by holding your pen with your arm stretched. Sometimes I use the edge of my sketchpad to do the same - the top edge for a horizontal reference and the side for a vertical reference.
If you're good at getting a sense of how each line or point relate to each other, you can even draw a perspective drawing without knowledge of perspective!