So what makes you a Sith Lord, isn't being the embodiment of evil enough?
"Sith Lord" is a title bestowed by a person with the proper authority. This would be someone who already is a Sith Lord, specifically the Sith Master (not just the Sith Apprentice) due to the Sith Rule of Two. There can only be two Sith Lords: if a new Sith Lord is appointed (by the Sith Master) when two are still alive then the old Sith Apprentice would lose the title.
Snoke and Kylo Ren aren't even Sith, much less Sith Lords. Even if they adhered to Sith beliefs they would need to receive the title from the reigning Sith Master: Darth Sidious. Both Snoke and Kylo Ren are users of the dark side of the Force, but not all dark side users are Sith (just as all Jedi are supposed to be light side users, but not all light side users are members of the Jedi Order).
isn't being the embodiment of evil enough?
Darth Sidious is still arguably the embodiment of evil in the first place. Moreover, even if Snoke and Kylo Ren are "embodiments of evil" in some lesser sense that does not necessarily follow that they are Sith Lords. One could argue lots of people are "embodiments of evil" for the evil they've done (e.g. serial murderers), even if they are not Force-sensitive. Of course such people -- especially if they are not Force-sensitive -- don't get to claim to be "Sith Lords" on that basis.
Null’s answer covers the basics for the current Disney Canon.
This answer is intended to cover the old pre-Disney lore based on the extended universe, now known as ‘Legends’, because it both gives much better context and is honestly much more satisfying.
In the beginning...
The ‘Sith’ were actually a species, not an organization. They originated on the planet of Korriban some time more than one hundred thousand years before the Battle of Yavin (BBY), were naturally Force-sensitive and particularly attuned to the dark side of the Force, had a very strict caste system, and viewed war and conflict as a part of life.
Around 30000 BBY, the previously highly tribalistic Sith were unified under the banner of Adas, the first Lord of the Sith (‘sith'ari' in the Sith language), with the title being simply descriptive. He ruled for at least 300 years, eventually leading his people to successfully repel a Rakata (another ancient force-sensitive species who ruled most of the galaxy between 28000 BBY and 25000 BBY) invasion, ultimately sacrificing himself to guarantee victory. Adas’s death caused the Sith to devolve into a constant state of civil war again, with many Sith trying to claim the title of Lord of the Sith. For thousands of years, they stayed mostly in their own little corner of the galaxy, isolated by a complex mess of navigational hazards that made it difficult, if not impossible, to leave their sphere of influence.
The next major development in the history of title began after a period of Republic history known as the ‘Hundred-Year Darkness’ (around 7000 BBY to 6900 BBY) when a group of human Dark Jedi fled Republic space (at the time mostly confined to the core-worlds) and eventually found themselves in Sith space. There, they essentially took over and inbred with the native Sith species. These Dark Jedi functionally re-unified the Sith, and took on the same title that Adas had millennia before, for essentially the same reason.
The Sith Empire continued under their magocratic rule for quite some time until the death of the Sith Lord Marka Ragnos around 5000 BBY, which lead to a vicious succession struggle between the Sith Lords Ludo Kressh and Naga Sadow that by pure coincidence was interrupted by a hyperspace exploration ship from the Republic. Sadow used the situation to pull an effective coup, which lead to a civil war within the Empire which Sadow ultimately won. Through some simple trickery he then leveraged the Republic explorers to obtain a route to the Republic, and he set out to attempt to conquer them as well. This failed miserably, the Sith Empire was dissolved and absorbed into the Republic, and the remnants were scattered.
Various remnants came back eventually around 4000 BBY, started a new Empire around a new Sith Order, went to war with the Republic, and failed miserably again. The series of wars this time are known as the ‘Old Sith Wars’, with the end of this particular cycle of wars covered in the Star Wars: Knights of the Old Republic video game and it’s sequel. In this particular instance, the Sith Empire was left much more intact than previous instances, but they retreated particularly far.
Then the same cycle repeated itself yet again about 300 years later around 3660 BBY, though this time the Sith Empire actually fought the Republic to a standstill, successfully sacked Coruscant, and forced a ceasefire. The Star Wars: The Old Republic MMORPG covers the final parts of the war. During this time though, the Sith were still the ‘ruling class’ of the (revived) Sith Empire, with anyone above the level of an apprentice among the Sith Order being addressed as ‘Lord’ by other subjects of the Empire, but the title (Sith Lord) was tied to personal achievement instead of heritage or practical position (in other words, they no longer bore the title because they ruled, they ruled because they bore the title). Because of the events of this war, the Sith species essentially disappeared from existence (with some limited exceptions), but their culture, artifacts, and teachings survived in good enough condition for the Sith Order to pop up again later.
And pop up again later it did, around 1000 BBY, sparking the ‘New’ Sith Wars, which ultimately culminated in the Seventh Battle of Russan. This battle wiped out a large number of the galaxy’s combat-capable Jedi at the time, but also essentially obliterated the revived Sith Order. The sole survivor was Darth Bane, who shortly afterwards instituted the Rule of Two that defined the Sith Order as it was until the defeat of Darth Sidious and the redemption of Darth Vader during the Battle of Endor. Darth Bane was where the final break came in for the title, he kind of defaulted to being a Dark Lord of the Sith because he was the only remaining member of the order, and from that point forward, the title was bestowed by the master to his apprentice.
Notably, this comes to essentially the same conclusion as the Disney Canon answer, after the New Sith Wars, you were a Dark Lord of the Sith because your elder Dark Lord of the Sith said so.