Step 1: Get yourself a measuring tape and take measurements of the inside of the tank.
Step 2: Search everywhere on the net for pictures and diagrams of sumps and come up with a design to fit your needs. Just to be nice i attached a picture of mine to get you started
Step 2: Find a glass shop and order acrylic baffles
Step 3: Pick up some aquarium sealant and get to work
Step 4: Figure out the max flow rate of your drain line (google search) if its a 1'' drainline than you can run a maximum of 600 GPH with a drill hole at the bottom of the tank and around 250 - 300 GPH if the hole is near the top.
Step 5: Get a pump to match.
You also need to look into standpipes/overflow boxes because that is something i never thought of when first getting my tank drilled and it served as a rather noisy mistake.
There is a crazy amount of information out there on this stuff. I would suggest that you do as much research as possible on as many different designs and concepts as you can. Don't take just one persons advice.