I'm certainly no expert, but I've documented my own process for writing and publishing content on DM's Guild, and I hope that my own experiences can off you some insights into what can be a confusing process.
DM's Guild Resources
The following items are content which I have published on DMsGuild.
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Character options for playing every monster in the monster manual. A Platinum best seller within 48 hours.
The sequel to Monstrous Races, Monstrous Races 2 converts every monster in Volo's Guide to Monsters into player options.
A collection on variant rules, including guidance on why you should and should not include each variant rule in your game.