If you think the plugin would be useful to other people, you can do a PR in nixpkgs, an example:
https://github.com/NixOS/nixpkgs/pull/127898/files
If the plugin is only for you, add it to your configuration. E.g. from my dotfiles:
Notice that src
can be also a local directory, that’s up to you
After that, is just another plugin, install it with home-manager like any other plugin:
programs.neovim.plugins = [ visual-star];