Ok, so if you have a sword for example in your first slot and the enchanted book in the second slot. You open your inventory with "E" a GUI
should appear, LEFT click on the enchantment book and hover your mouse over the SWORD while holding the book and than your going to LEFT click the enchanted book on the sword while in the inventory. And it should apply if it's an enchant that can go on that specific tool or armor.
For example sharpness on a sword instead of like efficiency because that won't work!