How do I get an even cylinder instead of one that bulges in the middle?

For the flat top of a cylinder, add 6 stitches per round: 6, 12, 18, 24, 30, 36. At 36 you have a disc whose diameter equals the tube's width. From that point on, adding any stitch makes the cylinder wider than the top, and the extra width has nowhere to go but outward.

The visual result is a ring of slack about 1.5 cm below the top, exactly where most people are still following 'increase in every round' out of habit.

After the final increase round, work exactly one round with no increases. This round absorbs the transition: the stitches in it lean slightly outward at the top and vertically at the bottom, forming the rounded shoulder a real cylinder has rather than a hard crease.

Then work straight rounds. Measure the cylinder's circumference after four straight rounds; if it is wider than the disc's diameter, you are still increasing and need to back up.

To narrow a cylinder — a leg going down to a foot — decrease on alternate rounds rather than every round, and stagger the decreases the same way you stagger increases. Decreasing every round at six points gives a visible cone; alternating gives a gentle taper over twice the height.

To widen — a body going into shoulders — add the new stitches one round late and one stitch in from the edge, so the widening happens inside the silhouette instead of at the outline.