Well to build your own case there really aren't any guidelines, just figure out how big you want it, how much space your components will take up, and what material. From there be creative, draw up a few plans see what you like. Make sure the case has good airflow so pretty much that its decently open.
Also make sure you're aware of any grounding points, particularly in relation to the motherboard. Making your own case can be a rewarding and effective option, but you may want to start by modding a cheap case, if only to get the feel of a dremel.
To be honest,.. i would look for a cheap 2nd hand ATX case. you can throw off the fron, and sidepanels, and muit those yourself.. but at least you can be sure you have the ATX format right