As of October 8th 2016, after roughly two years of offering free critiques, I will now be limiting my own critiques to patreon supporters ($3 or more). But don't fret - you are still welcome to freely submit your work and questions directly to the /r/ArtFundamentals subreddit to be reviewed by the community.
If you haven't yet completed Lesson 1, go back and do that first. This exercise is best done anywhere after lesson 1 and before lesson 6.
Similar to the 250 box challenge, now it's time to draw 250 cylinders. It's pretty much the same concept, and if you haven't done the box challenge, do that first. Vary them in size and length, and before you start, read the "How to Draw a Cylinder" section below.
This should be done in ink, preferably with a fineliner/felt tip pen.
I discuss this in the video linked above, but I will summarize it again here.
This is the most important part - you start with the minor axis. If you don't remember what a minor axis is, jump back to the ellipse section of lesson 1.
Using this line, draw your first ellipse. I generally start with the one closest to the viewer, but it doesn't really matter. Just remember that the end closer to the viewer is going to be narrower (have a smaller degree) than the other end.
Furthermore, remember to align this ellipse to the line you drew previously - as the line is the minor axis, it should cut the ellipse into two equal, symmetrical halves down the narrower dimension.
Next, draw out the two sides of the cylinder, determining its length. There are two important things to remember here:
Finally, draw the ellipse at the opposite end. As I mentioned before, the degree of the end farther from the viewer is greater - this means it's going to be fatter. Due to perspective it will still be smaller - you must understand that these are two different characteristics, and that an ellipse can be both smaller and fatter at the same time.
As I stressed in the previous section, it is extremely important that you use the central line as the minor axis for both your ellipses. If they are not aligned correctly, your cylinder will simply fall apart. Now, if you want to draw a form that is like a cylinder that has been cut at an angle, rotating one of the ellipses (or more accurately, its minor axis) is a great way to achieve that visual effect.
Ensure that the ellipse farther from the viewer has a larger degree - meaning, it is wider.
Ensure that, as according to the rules of perspective, the ellipse on the far end is smaller than the one closer to the viewer.
This one is taken from the 250 box challenge, as the same concept applies in all drawing.
As you get more confident with drawing your forms, you can start to play with line weight. By default, we draw all of the lines of a box with the same thickness. This results in a box that feels somewhat more like a collection of lines, rather than a single cohesive form.
The first thing we can do to make it feel more cohesive is to add weight to the outer lines of the box - that is, the lines that define the silhouette of the box. Turn off your 3D-thinking for a second and look at the box as a 2D shape. Imagine if it were completely filled in with black - that would be the silhouette.
By thickening the lines that define the edges of this silhouette, we start to establish a hierarchy of importance. The heavier outer lines establish a grouping, and the lighter internal lines become subordinate to that. It starts to feel as though the overall box is a single cohesive unit, rather than a collection of barely-related marks on the page.
Now, thickening the line all around the box is definitely good, but it also causes our box to flatten out somewhat. We're focusing heavily on the 2D silhouette that it suppresses the 3D nature of the form.
Luckily there's something we can do about this - we can thicken the silhouette-edges of the box, but we can do so more in some places and less in others. Think of it as though the lines of the box are actually little shadows, and you have a light bulb hanging somewhere near your box. The lines that face away from the light source are going to have larger shadows - so they'll come out a little thicker than the lines that on the side of the box that faces towards the light.
It's no doubt a simplistic way to think about it, but it will help you develop more dynamic line weights.
Line weight is an extremely useful tool to establish a hierarchy in your drawings. Often times it is this notion of organizing your lines that sets apart a very boring, stiff piece of line art and one that feels dynamic and alive.
If you've taken a look at lesson 2's form intersection section, you'll notice that we discuss the concept of "dominance" between two forms. To put it simply, we're talking about one form overlapping another. Line weight is extremely useful in this situation, as a thicker line automatically feels like it is in front of a thinner line. So, in this example you'll notice that the long box's bottom side has a very thick line to it, which gets thicker as it passes over the other box. This variance implies to the viewer that it has dominance, and that it is closer to the viewer.
Remember that line weight is a very flexible thing - even the same segment of line can gradually grow thicker or thinner over the course of its length, and this tapering effect gives a sense of fluidity and flow to your strokes that helps keep your drawings from looking stiff.