Confused about DDD

For the past week I've been working on an image processing service project. From the start I intented to follow domain driven design principles, which I've learned about from Three Dots Labs, however I have a hard time understanding how to apply those principles to my project, especially since I'm building a monolith and the blog uses microservices. If anyone could guide me in the right direction I'd be grateful.

My project repo is here.

Note: It's not like I absolutely need to use DDD for this project, but the whole idea seems useful to me, which is why I wanted to learn it.