Hello!
To use spell:
(In the spell menu) left click on a spell to select it. Then, click on a slot to bind it. After that, (in the main scene) left click on the slot to select the spell as the current spell. Left click on the enemies to use the spell.
Merging feature:
(In spell menu): Left click to select the first spell, Right click on another spell to select it too. Then, press the Merge button. It will create a custom spell, combining the effect of the spells you selected. Unless you buy the right items in the shop, you won't be able to use any custom spell for merging.
Hope it is more clear.