Once you installed the plugin, Go to the Post timeline Plugin under the WordPress Admin Dashboard. You have two options to add Timelines which are mentioned below;
1- Click on the “Add New” option under Post Timeline Plugin
2- Or You can Click on the All Timelines Tab under Post Timeline Plugin
3- Next to the Post Timelines title, you can see the Add new button on the right side of the Plugin, Click on it to add new timelines.