Did you download the demo project and read the docs ?
First set your slots to run as “Synchronous Task” and stop using the “Async” nodes if you feel lost. Use default non async nodes first, it’s easier for non programmers.
When you execute “Load Game World”, if the level is not the current, the plugin will first open the level before loading data from Slot. See demo project for reference.
The properties you want to save are only saved if they are marked with “Save Game” checkbox on advanced section of the property, and your instance has a valid “SGUID” Guid property in place. Again, see demo project for reference.