You craft them with either Vines or Moss, the same as vanilla blocks. Pretty simple!
Double-Click the "MoreMossy_By_JayCubTruth.mcaddon" file, and it should automatically open and import to minecraft. Once imported, you can add it to your world by going to the world settings and clicking behavior/resource pack in the lower-left corner. From there you will click on "my packs", select More Mossy by JayCubTruth, and select "activate".
Next, you must make sure that "Holiday Creator Features" are turned on, or it will not work!
You can find them in the "game" section of your world if you scroll down. Once that's done, you are ready to go!