How to create an eternal world?

Recently I've seen a lot of people talking about an "eternal world", where you would play for a long time, kind of eternally. But I have difficulty creating one, since every time I try I end up giving up. The most I've managed to do was get the elytra, but after that I ended up having no idea what to do. Can you give me tips on how to make an eternal world? Preferably those who have one, also tell me if it is a worthwhile experience or not.