First thing you need to know is how the power supply works. Black is - red is 5v dc orange is 3.35v dc and yellow is 12 v dc. The best way to attatch wires is thru sata or 4 pin molex connectors easily available thru local computer stores or e-bay. It is also crucial to not over draw your power supply and burn it out so plan the wattage use as best you can ,thus saving you from having to buy a new power supply when yours burns out .
Next part is knowing what it is your going to do. I can not emphisize how crucial planning is to creating a nice clean and easy to assemble case mod.
Last but not least shop around some people will charge you way too much for components search the web, browse locally and set up a shopping list befor you purchase. I was able to do a complete internal mod with cut in window for under 100 bucks Canadian. Thanks to shopping and looking around.It would have cost me 450 ish dollars if I would have paid full retail price to accomplish the same outcome.
Hope this helps and happy Modding!