Simple, by bearing witness to multiple terrible DMs that have been in my group, while also seeing people actively enjoy when I DM after I started a campaign on a whim after a DM quit our group. I just went "Well, I can't be worse then That guy right?" and ran with it, jumping into a homebrew campaign with a homebrew world, both of which had 0 planning and about an hours worth of thought put into it in advance. Even the map was something made in MS paint about 10 minutes before the session. Everyone had a blast and I have been Forever DM ever since, even though I still barely know what I am doing.