You can either add a new ressource to the Resource Manager (click HERE), or make your own thread with all your imports in the Imports section. In my opinion, the best is to do both, with examples of your mods in your import thread along with links to the Resource Manager.
It is important to add a screenshot of your creation, so the users can see what your mod looks like, and possibly give the Custom Data if you want to add clothes or eye colour to your character for example.
Look at one of my mods on the RM to see a good and simple way (I think) to present your work.
For the screnshot, I advise to make a square one (edit it with Paint for example) so it can be used for the example and the icon. Upload the example picture to undertow.club, so there is no risk that it will be deleted on some random image hosting site ("Attach files" button).
For the Custom Data, edit it with this tool that removes the unecessary parameters and put it inside CODE tags (click the three dots near the smiley in the top of the edition window and choose "</> Code").
At last, remember that even if you don't sell your mods, it is important to take some time to present them well, because even a very good mod with a bad presentation will only be downloaded a little, and it is a bit stupid to spend time on something that will not be used much.