A sump is nothing more than a second fish tank that is plumbed into the first tank and normally hidden to conceal all the filtration garbage.
Think of a public aquarium display.
The trick to sumps is the design and safety features -these can make it or break it. The hard part in DIY sumps is the plumbing-this is not an issue with premade sumps. I would not recommend making your own sump until you have had one. The design looks simple when you understand the mechanics, but thinking it is will be the fastest way to soak your carpet.