Paint, stencil, or freehand draw.
Lots of choices. What do you have now?
Steel, aluminum or wood?
What are your skills and access to tools?
What does the rest of the house look like?
Hello, a Photo or two would help with answer..........
Paint it you could paint fake Hal moon windows on the top it looks great it looks real
Paint is the easiest option.
Here's a video that might help you - https://www.youtube.com/watch?v=XQyqA0iC3_Q
This is a good one: https://www.hometalk.com/27883673/garage-door-makeover