Step 4

For curves, I like to create custom brushes.

Rather than eyeballing line weights, you can set up Variable Width Profiles that automatically apply to a line.

Select the Line Segment Tool (\) and draw three to five lines, holding down the Shift key to keep the line fixed horizontally.

Then select the Width Tool (Shift-W) and create different width profiles for the strokes. This will enable you to adjust the settings that you want the profile to duplicate. 

Next, hit the Add to Profiles button at the bottom of the Profile panel, in the Stroke panel.

This will pop open the Variable Width Profile window, to enable you to name the stroke. Click OK.

Delete the custom line paths created after their profiles have been saved.