A recent Wizard’s article, Unearthed Arcana: Encounter Building, states:
If you want to use a single monster, you’re typically best off using a legendary monster, all of which are specifically designed to make life interesting for a group.
The article identifies creatures of CR 1 as being an appropriate “Solo Monsters” for four first-level characters, for example.
I like the article’s advice, but the legendary creatures in the Monster Manual are mostly at least CR 10. There are no good examples of balanced, low-level “legendary” creatures, for solo fights.
What the core rulebooks say
Checking the DM Guide rules for Creating a Monster (pp. 273–283) I don’t see guidelines for customizing a creature to be legendary, or for creating one from scratch. And the Monster Manual section on Legendary Creatures (p. 11) describes legendary actions or lair actions, but does not provide guidance on designing your own.
Any Good Solution is Welcome
If there are guidelines from Wizards somewhere that I’ve missed, please post that link. Play-tested house rules would also be great. I have seen a third-party article or two out there — if folks have play-tested these suggestions, your impressions on them could also be a good answer.