How to make a fantasy map sketch look professional?

[TimPaul]

The best way to get something to look professional.

Find some professional looking maps. Then practice making maps till they look as good as the maps that inspire you. I do not mean copy those maps, or imitate them. Figure out why they look professional.

How do they handle text. How do they handle color.

Then practice, practice, practice.

[Falconius]

I do not mean copy those maps, or imitate them.

If you do copy or imitate them you also pick up skills as well. Many, many illustrators started out copying their favourite stuff. You can also embellish your favourite works if you go down this route, see why what they did works and why they left this or that out. Etc. In short, don't be afraid to copy to improve your skill.