Third tip:
Aesthetics matter. Whether playing via web or email, it's likely that your playtesters are going to spend a lot of time looking at your map. You don't have to be a classically trained artist, but you shouldn't expect a slapdash approach to be sufficient.
While there's a certain consistency to the style used at vDip and webDip, it's not the only one available. I'd suggest browsing through the archives of Play-by-Forum games over at PlayDiplomacy. You'll find quite a few varieties of style in the maps and creations used.
-- http://www.playdiplomacy.com/forum/viewforum.php?f=87
Fourth tip:
In any case, you need to understand at least 1-2 graphics editing tools, and they need to have support for LAYERS. I cannot stress the importance of this enough. Layer-based editing tools allow you considerably more freedom to correct mistakes and make changes. GIMP and Paint.NET are two of the easier tools to work with for basic pixel-based graphics.
-- https://www.gimp.org/
-- https://www.getpaint.net/
Now if you REALLY want to dive into the deep end, you can learn how to work with vector-based art (SVG format and similar). Adobe Illustrator is the absurdly expensive industry standard, but Inkscape is completely free and can handle the task:
-- https://inkscape.org/en/
(Inkscape's only real downside, other than the learning curve, is that it can get pretty slow when you start working with immensely complicated geometry. It renders entirely via CPU rather than splitting the work between CPU and GPU, which can make it frustratingly slow even on really good PC's. However, as a beginner, you will likely not encounter that problem.)
Bonus tip:
PlayDiplomacy's Play-by-Forum community is a really good testbed for new variants, particularly when you're working with more than just the vanilla ruleset. There are several variant designers who play, create, and administer games there (including myself).
-- http://www.playdiplomacy.com/forum/viewforum.php?f=34