How Do I Do That in Photoshop?

Draw a Single Thin Line of Pixels?

You could use the Line tool, and set it to a Width of 1 pixel (see page 40), but for stuff like this, you might want to use the Pencil tool (it’s nested in the Toolbox with the Brush tool; or press Shift-B until you have it). It draws a 1-pixel hard-edged line, by default—you just get the pencil and start drawing free-form with it. If you want to create a straight line, press-and-hold the Shift key first, click the Pencil tool where you want your line to start, move your cursor to where you want it to end, click ...