There are two basic methods to create 2d animations in any 3d application:
- model regular 3d characters or objects, texture, and add rigging to animate. Then animate, make sure things loop (if required), and render each frame in a sequence of files. The final result can be more realistic looking, or by utilizing a non-photo realistic style, can look like a 2d drawn result.
- draw your characters in an illustration package (such as InkScape, Illustrator, Flash), segment the characters into animate-able parts, and import each part as an image projected (uv mapped with a texture) on a flat 3d plane. Then rig the parts together for an animateable character (or object), and animate. Render each frame in a sequence of files.
The first method is more time-consuming, because you will need to model, texture, and rig a full 3d character. But if you require lots of turn-arounds, and full flexibility, this may not be possible/feasible with the second workflow. There are thousands of tutorials that demonstrate the concepts of building 3d textured animate-able characters and objects out there. For Modo, no so many, unfortunately.
The second method is very doable, and a lot of fun. Here are some examples:
https://www.youtube.com/watch?x-yt-cl=84503534&x-yt-ts=1421914688&v=VvbhfLBIjHI
There is a nice thread on BlenderArtists, and you can even download a fully rigged 2d character for Blender that you can examine, and use as an example for Modo.
http://www.glennmelenhorst.com/misc/ble ... _028.blend
If you need more assistance with this, I recommend posting a question on the Modo forums: that community is very helpful.
Also, if 2d flat characters are your goal, spend some time investigating Spriter: it will bring in fully animated characters with a rig right into Construct2. Rather similar to the second option I mentioned, but in this case the rig is retained in Construct, resulting in much smaller file sizes.