Goldfish, I understand your confusion on the sumps. Some times it can be hard to visualize how things work or go together until you see one operating. If your LFS
has setups with Wet/Dry filters or sumps you might be able to talk them into showing you how their setup works.
To go from the tank to the sump you need to first to thru the overflow then thru tubing or pvc
pipe into the sump. From the pump you would go into a true untion (so you can disconnect the pump from the plumbing) and then thru a ball valve (so you can turn off the water flow before disconnecting the pump). If your pump is external then you need another true untion and ball valve before the pump aswell.
From the ball valve you would use PVC
to go up to your tank. and then use PVC
elbow fittings to go over the lip of the tank. Size the PVC
you need based upon your pumps outlet size. If your pump outlets on 3/4" fittings then use 3/4" true unions, ball valves, and PVC
fittings. All the ruturns I have ever built I use 3 90* elbows. Two elbows connected with a short bit of PVC
will do a 180 up and over the lip of the tank and then another elbow will shot the water out into the tank just below the water surface. Drill a few small holes below the water surface on the returns to give it a syphon break for when the power cuts off. If you dont then water fill back syphon until it reaches the top of the highest return. If your returns are 1/2 down into the tank thats alot of water flowing back into your sump till the syphon can be broken with air.
Its also important to note that you can not run the sump full with water in normal operation. A full sump when the tank is running is often only 1/2 or 1/3ed full and when power is cut off you have a fair amount of area for the back syphon of water.
You should be able to find some pics of sumps and return lines in the Members tanks section of the photo gallery.