Arcane University: Mesh Export to NIF
In order for your mesh to be used in Skyrim, it must be in .nif format. There are several ways to do this, depending on what 3D software you use.
Blender Niftools Addon
You can export directly from Blender using the Blender Niftools Addon. Which version you should use (and how you export) depends on which version of Blender you are using. It allows for directly exporting from Blender to a full NIF, but you will need to merge the created NIF with separately created collision using ChunkMerge. The advantage of this method is that you do not need any intermediate files and can export directly from Blender.
The process for direct export from Blender 2.7x is documented on Blender 2.7x Export.
The process for direct export from Blender 2.8 or up is documented on Blender 2.8+ Export.
3DS MAX Export
The process for direct export from 3DS MAX export is documented on 3DS MAX Export. It allows for direct export from 3DS MAX to a full NIF. The advantage of this method is that you do not need any intermediate files and can export directly from 3DS MAX. The disadvantage is that it does not work for every version of 3DS MAX, and will therefore need to be updated when updating 3DS MAX.
CK-CMD is an independent command-line software you can use to, among many other things, convert FBX files to NIF. You can download it on its GitHub page. To convert an FBX file to NIF, first you must export from your 3D software with the correct settings. Firstly, make sure to apply transforms before export. Because Skyrim uses a -Z Forward, Y Up coordinates system, you must export with these settings or else your mesh will be flipped on multiple axes. Once you have exported your FBX with the proper settings, in the command line run the command
ck-cmd.exe importfbx (FBX filepath) -e (output folder). This will create a NIF with the same name as your FBX in the output folder specified. It should be noted CK-CMD exports Legendary Edition NIFs, so if you want to use your NIF in Special Edition, it must be converted first with SSE Nif Optimizer.
NOTE: If you don't want to run the command line every time you want to convert, you can create a file in the same location as ck-cmd.exe which will convert just by dragging and dropping FBXs onto a .BAT file. To do this, create a .BAT file (Notepad++ supports saving in this file format) with the code
"%~dp0\ck-cmd.exe" importfbx "%1" . > logfile.txt 2>&1. Once you have saved this, you can convert to NIF by simply dragging and dropping your FBX onto the file you created.
The process for exporting using Outfit Studio is documented on Outfit Studio Export. The advantage of this method is that it allows for exporting for any 3D program which can export to OBJ, as well as not being dependent on that program's version. The disadvantage is that you have to work with intermediate files, which makes it harder to quickly test out small edits in-game. Furthermore, any rigging will need to be done within Outfit Studio, as OBJ files cannot contain rigging information.