Custom enchants in a loot table

So I have struggled to add custom enchants into a loot table for a while now. So if you want to add one what you have to do is create entry and selct book. This has to be a regular book not an enchanted book witch is the most confusing part. So what you want to do is find the range of anchantments. So you have to find the type you want (example MyCustomEnchantment has a level of 10 in the level 3 is 30 enchant levels youd select range 10-30 so put both in range.) So once youe found it you need to go below th item and slect the range.