1> Create the mod

2> Go where you saved the mod files .
3> Compress the mod files with your favorite program, just the files, not the folder where the files are!!! Otherwise the game won't be able to find it, however you can put any eventual screenshot or readme inside a folder.
Ideally the archive should be structured as:
5098304982.png
982409348x.png
MyMod (folder) ----> mymod_readme.txt , mymod_screenshots.jpg
4> Now you can upload it to the Nexus and enable "Download with Manager".