At the most basic level if you have a drilled/RR tank the overflow will have a bulkhead fitting in it...most are drilled on the bottom and use a durso standpipe
but some are drilled on the back and use an overflow box. To the "bottom" side of this bulkhead you will attach some type of plumbing (hose/tubing, spaflex, rigid PVC) so gravity can carry the water to the sump.
Sumps have too many designs to go into, you'll have to look around and see what you like. I personally like a simple design with enough room to house heaters, skimmer, etc. From there you will need a pump to get the water back up to the display tank...this can be a submersible or the sump can be drilled and use an external pump.
As for the RO/DI and top-off you'll want to setup your RO/DI as close to the tank as practical so you don't have to lug water any further than needed. Top-off water can be added manually each day, but an auto top-off device is very nice. I just have a container that holds several gallons of water with a float valve on it which sits on top of my sump, but that's in my basement fish room with tons of extra room. I like this method as it's pretty fail proof and even with a total failure it can only add a few gallons of fresh water to the system. The other option is a pump fed ATO. If this option works better for you you'll need to look up some designs. Most people set it up with 2 float switches, but again there are all kinds of designs using solenoids etc.
Lastly most people do water changes via buckets, trashcans, etc. You'll most likely want to siphon out the old water from the sump then siphon, pour, pump, the new water back into the sump. This way the water level in the display remains constant. There are various ways to make the water changes easier/semi-automated, but unless you have a dedicated room behind or below your tank most will not be an option.
Some of that may be overly simplistic, but hopefully it is helpful.