F&F comes loaded with all of the spells found on Open5e. For anything not in the D&D SRD, you can add custom spells in your Workshop, Worlds, and Campaigns.

Campaigns

You can access your spellbook by either clicking the wand icon next to the chat input, or the spellbook tab in your character sheet.

Workshop

From the workshop, you can create custom spells which can be imported into all of your worlds.

Worlds

From the Spells tab of your World, you can see and create custom spells. These spells will also be present in the world tab for any campaign played from your world.