It depends on the complexity, but spells can be pretty simple to make. Basically you set up a GPL function that the spell will call when executed.

Normal Spell:
Define spell in Action.xml -> Spell calls GPL function to execute

If you want it to be a projectile, you define the projectile in an XML file and do the same thing as above.

Projectile Spell:
Define spell in Action.xml -> Spell calls the projectile, defined in Projectile.xml -> Projectile calls GPL function to execute.

I recommend just looking through Spells.gpl file as that's where most spells are defined and follow them through their GPL function(s), and see how they are set up in Action.xml and Projectile.xml.

Most of the spells I've made, I've just modified existing ones like Chain Lightning / Lightning Strike so that they work with characters using them, but I haven't modified the effects or damage of them so my knowledge of how to make a spell like that from scratch is limited.

Easier spells to make are ones that just create units, as that just means you have the spell call a GPL function that does some SpawnUnit a few times.