Just download the package - unzip it - open the project - select one of the grass meshes - right click on it - migrate - choose your project - ok. Or you can also move all the folders to your own project -> but you will have to keep the folder structure!! Otherwise you will loose the references to the assets.
When you want to use the “pro” version of the materials, you will also have to copy the blueprint part of the character bp into your own char bp 