You need an external overflow box and a return pump (and all of the plumbing in between). The plumbing will be the most challenging part, but it is fun and interesting too.
I use Marineland SOS overflows and like them. I hear Lifereef makes good ones too. I have heard mixed reviews on the CPR brand.
I use a Dolphin return pump and like it. Many people here like the Mag pumps also. Mine is submerged in the sump, after some acrylic baffles I siliconed into the tank.
I have two overflow boxes (125 gallon tank), one tees off to a refugium that is on a shelf in my cabinet. The other goes directly to the sump. The refugium gravity overflows into the sump through two bulkheads.
I use a combination of pvc spa flex (white) and reinforced vinyl (clear with embedded threads) with standard rigid pvc attachments (elbows, tees, etc). I used pvc primer and glue to attach them. There are barb fitting you may use to put together the clear tubing, just remember to cable tie or clamp them. I chose not to use them to avoid reducing the tubing size at the joints (but, many people use this way and like it). All of these can be found at Home Depot etc. Look for the barb fittings with the lawn sprinkler/ irrigation supplies. Flexible pipe/ tubing is quiet and easy to work with/ modify/ move around.
I installed a pvc ball valve in the return line just after the pump. This is to adjust the flow from the pump. My pump is pretty strong and so I need to cut back the flow a little to balance out the water level so that my overflows keep up properly. This is a trial and error thing that feels out of control at first, but you get more comfortable with it as time goes on. I also put a swing check valve in the line between the pump and the ball valve. This helps prevent a flood if the power goes out and the water flows back into your sump.
The main flood preventions are the anti-siphon holes in the return lines. Just drill a ~1/4" (small but not too small so that it will plug easily) hole in the return line, just under the surface of the water/ water line. Water will spray out of it and ripple the surface, but this is a good thing. Then, if the power goes out, water will travel back through the return lines to the sump (and yes, through the pump). When the water level drops below the anti-siphon hole, the siphon that pulls the water down to the sump breaks and the water stops flowing. The check valve at the pump is just a redundancy.
There are many designs posted on various threads here. This is just mine. Do forum search and see the related posts from other threads to see the others. You will get alot of info fast that way. Either way, you will need the same equipment listed above and the principals are the same.
Hope this helps.
Good Luck
-Christine