Paint the bottom of your door first, this way you do not get any paint on your floor when you apply the rest of the paint with the paint roller.
If you are only painting one side of the door, make sure that the side of the door you see when you swing open the door is the same colour as the door panel.
Once you have all the corners and edges painted, now you can start painting the rest of your door! Easy right? Using a paint roller, start by doing horizontal lines first, and then finish with the longest vertical lines for the cleanest finish.
Once you have one coat of paint done, be sure to repeat this process and do a second coat. This will ensure a smooth and blemish-free finish.