I'm currently putting a navmesh in a newly created level. For those who don't know: A navmesh is a part of the level that is invisible to the player. But it tells the NPCs where they can actually walk and where not. So if you have a room with two doors and some furniture in it, and you want an NPC to be able to successfully navigate through that room, you need to lay out a navmesh on the floor. It has to cover the free areas and avoid the furniture.
The Creation Kit actually has some tools that can automatically generate a navmesh. However, I found the results I got with them less than ideal. So I'm usually making the navmeshs by hand now.
And let me tell you: It's not exactly the part of modding that is the most fun.
What you see on the screenshot is obviously not the final result. It's a work in progress. In the end, all those meshes have to be connected and the space in the middle neeeds to be filled as well.
By the way, the screenshot shows a level from the new content that is not part of the currently released version of TIR. :)