To learn and cast a spell, a
Favored Soul must have a Charisma score equal to at least 10 + spell level.
Regarding Charisma, the 32 point version of the build makes the assumption for the following 'static' level 20 stats which lets you cast your max level 9 spells but doesn't spell out how how to be able to cast all your spells as they become available on your way to 20:
Cha = 9 (base) + 3 (tome at level 11) + 2 (capstone) + 6 equipment = (20)
This allows you to cast all your spells at 20 but you are correct that it makes it a little tough to get there...
When min/maxing with the build as is, an easy to get +6 item will only get you up to level 5 spells and thus the +3 tome at level 11 (or at least +2 at level 7) is pretty much mandatory for the build though that will still only get you level 8 (7) spells until you get to level 20 and get the capstone and it would suck to have to wait for level 9 (8-9) spells till 20.
To compensate while leveling you could take the FvS Charisma Enhancements as soon as you can at levels 2,6,10 and as a HE you can take Human Adaptability Enhancement at level 5 to bump Charisma by +1 as well and then you can reset these enhancements to the build's recommended enhancements when they are no longer needed.
You will also want to watch the Auction House for Charisma items and keep that maxed and trade up as fast as you can to +6 at level 11, 13, or 15 (can't remember for sure off hand and if you can find a Race Restricted one that matches your race you can equip it two levels earlier than normal).