When creating my pixel outlines I use several methods. Sometimes I create my outlines by opening a new blank file with a white background and I paint and erase one pixel at a time until I get the image looking the way I want. That is what I did in creating the froggies used in the sitting and jumping froggie tutorials. When using this method if you set your colors to black and white with black as the foreground, it is easy to "erase" unwanted pixels by right clicking to cover them with white to match the background.
Other times I begin drawing my outlines by setting my foreground color to gray and pressing and dragging to get an aproximate shape. Then I create a new layer on top of my digital sketch and I pixel the outline above it in black. In this method I use the eraser tool to eliminate pixels I need to erase, as using white would hide the sketch on the underlying layer.
Another method I use is to draw the image I want to create with paper and pencil. I then scan the image at various sizes. Once I've done that I crop it to only include the image I'm creating the outline for. Then I decide exactly how many pixels tall I want the image to be. For my pixel outlines this is usually 60 to 100 pixels but it can be as large or as small as you like. At that point I do a resample and set my height to the size I've decided. If the image doesn't look like a good size then I will undo and resample again with adjusted settings.
Once I've gotten the scanned image to a size I like, then I create a new layer above it and start painting and erasing until I have the outline I want.