You can paint a front door without removing it. But you do need to remove or tape up the hardware to paint more easily.
Have a plastic front door? You need Slick Stick. A product to prime shiny surfaces. Two coats of Slick Stick and you can paint your color on top.
Paint the details first. It's best to do this with a brush. Then smooth out the brush strokes with a roller. Two or three coats of paint is needed for full coverage.
For a long-lasting finish, seal the paint. Apply two coats of top coat to withstand the elements and daily use.