Pullover hoodies always mess up my hair!! I want to add a zipper.
I'm going to use the pencil tool in black to add a zipper. I'm going to make sure Y symmetry is turned on - either by pressing the "Y" key on the keyboard, or hitting this button in the top toolbar:
Then I'm going to take my pencil and run in down the middle while holding the SHIFT key. This makes the line I draw stay perfectly straight! Bam! My zipper looks good.
Then I'll zoom in using the SPACEBAR key, and erase some of the original lines to make my zipper look like it really belongs. I'll even draw a little pull.
Then I'll draw my ram horns!! Since symmetry is still on, SketchBook will mirror my horns on the other side perfectly.