How do you market a Figma plugin?

You create a Youtube channel about the tips and how to dos in Figma. You put so much content there that every person to ever use Figma had checked your channel at least once. Free figma courses might also be very smart but if you are doing courses you might as well sell it on Udemy and make some money at the same time... Also you can do hackathons that focus on designing processes.

Think of where your Figma users are. The YouTube tutorials suggestion from @tunc_emran is great, I would also explore online communities on Reddit, Facebook, Slack, Quora and so on and post tips as well as casually promote your plug-in. You can also create a lead magnet full of helpful things and drive traffic there from social media or make it really SEO optimised. You can then grow an email list and sell to them.