The key to the clean lines of this Halo symbol is an x-acto knife. I have one that stays in my cake decorating toolbox for this very purpose. It allows me to cut the fondant exactly the way I want it.
I really enjoyed painting these scenes from the video game onto the cake. I first cut the panels out of white fondant and attached them to the cake. Then used gel paste food coloring mixed with a tiny bit of vodka (you can use any clear alcohol) and used a small paint brush to design these. This paint brush stays in my cake decorating toolbox too. Tip: when you're painting fondant, you only need a tiny tiny amount of alcohol to thin the food coloring. It evaporates very quickly so it doesn't leave your artwork looking wet (water doesn't evaporate as quickly so it doesn't work as well for painting).