The application allows to install mods by modifying pak files which is against the EULA, so do at your own risk. The limitation is that you can only install a few mods for the same pak file without the anticheat prompting with an error. For more details on limitations of modifying pak files join Mod By Daylight discord server https://discord.gg/aAB2fecQtm and see #textures-pakediting.
Json mod package file is the only file you need to install a mod, no need to include ubulk files.
Before installing or uninstalling any mod you need to set path to the Paks folder. When the path is set, click on the install/uninstall button and choose json mod package to install/uninstall.
Click on 'Create mod package' button to open the creator dialog. All fields are mandatory, you need to provide mod name, pak file name, path to the original ubulk and path to the modified ubulk. Click 'Create json' button to open a dialog to save the json mod package.