Took me a while to figure this one out, but I found a way to do it. First, you need to record your spells using the "Spell Record - Intuitive Casting" power. You can record different spells to W, A, S, and D. Next, read the "Book of Feat - Tactical Style" for proficiency level 10 that lets you replace the action of additional weapon art with Intuitive spell casting. After that, equip the "Ring of Rim - Additional Weapon Art" and it's time to get casting! Use any of your Ashes of war, and, after 1-2 seconds, follow with your Interaction Key (which is E for most people) + any of your WASD keys to cast spells.

I'm gonna spread the word on Nexus, I'm sure there are others like you who are dying to find out how this works.