Help with creating spells on the map!

Since no one has answered, I will. You can't create your own spells, at least I haven't seen anyone do that. You can modify spells, in the sense that a spell might end up in a different school of magic, deal different damage, cost a different amount of mana, etc. The map will become a mod. What was shown in the video you mentioned is just pre-rendered effects; they have nothing to do with the spells on the map.