Briana, welcome...
All of the above comments you have so far are great, true and I agree with them...
I would like to add to them by noting, the suggestion of... Look at those 'guidelines' and think "shapes" when looking at your subject. Try not to re-create the images in the book, but rather... apply some (not necessarily all at one time) aspects or techniques to your composition. I have mentored young and old, as well as worked in tandem with artists of varying skill levels, and I can say one thing is common... We artist are visual. Seems dumb in its simplicity, but true nonetheless... If a tree looks like a triangle, or a circle... start there as your guidelines..
Using a landscape as an example... if some trees seem farther away than others.. then draw a horizontal line across your paper or canvas, to establish the horizon... then put a little dot or mark.. of the furthest point that you would be able to see, on or slightly above the horizon line.. Then decide... Do I see the top of my closest tree... if so.. make a mark at the top of your paper/canvas on whatever site it is... using a ruler... make a very light line that connects these two marks... Do the same kind of thing with where you think the roots of said trees will meet the ground. You may not see the closes trees trunk.. so pick the next tree in line, etc.. connect this mark to the horizon line.. (if your initial mark is above the horizon, make the root line directly below on the horizon line)...
Now.. add the trunks... start with only a few to get the hang of it... On closest (full heigth).. next somewhere in the middle.. and one really far (closed to that Vanishing point). if pines. block them like triangles.on sticks (your trunks)..
My best advice is play with it... no matter what terminology or technique you use, experiment. Keep it simple... Do things over and over, try differnt methods... Perhaps Perspective and proportion guidelines aren't for you and you are more of a contour artist... Experiment with that!!
There really is no Wrong way.. and every artist tackles the same things a little differently... Do whatever brings you to a happy point with your art, and is enjoyable in the process.. When both are met, then you have what works best for you.
All the best!