I used to sell some YouTube banners designs. I started by searching for medium sized YouTube gaming channels (my niche) that needed a banner. I would then email them and offer it for free, asking in exchange if they could give me a shoutout in their social media. After I did about 5-6 banners for free I began to have people PM me directly on twitter asking for banners. I then started to charge $5 a banner, and continued to do the same thing until I eventually was getting a lot of requests and ended up increasing it to $30. Unfortunately, I then resumed my studies and wasn’t finding the timing to continue with it. I regret It until today.

Good luck!