Mixing 2D animation elements over 3D isn't a new technique and has been used by traditional 2D artists for dynamic shots in many old Disney films. However, with movies like Spiderman: Into the Spiderverse showcasing how amazing and modern the technique can become, the revival has inspired many to take to it yet again. Now, 2D animation is often used alongside 3D parts and blended seamlessly so as to seem like a whole.
Here are some simple tutorials to get familiar with the technique. Follow these for a quick intro to the process.
Comprehensive intro to using 3D and 2D in Blender
This creator doesn't use Blender, but his insight to the process is unparalleled. Watch to get an idea of how large scale this technique can become.
This video is a brief overview of a pipeline including both 2D and 3D elements
The technique is unique, and can be manipulated to fit Blender if you are up for a challenge.
Create a short animated clip utilizing 3D and 2D elements that lasts at least 10- 30 seconds. Remember to get as much inspiration as you can from existing projects, and try not to attempt too grand of an undertaking at the first attempt.
This pipeline may help you break down the stages for your project with more visual cues for each stage.
Start with a rough sketch of the idea in your mind. It is not an exact vision of how things will look, but it does try to find how you will present your main character, how you will place some elements and what mood you want to convey. This stage describes it more like making a storyboard for a movie or a concept art for a scene.