[MENTION=6923088]Aebir-Toril[/MENTION] pretty much has it. There are two main things that make a legendary monster (though not all of them follow this path explicitly):
1) Legendary resistance: succeed on a certain number of saves. I sometimes make it unlimited saves, but cast a reaction
2) Legendary Actions: usually 3, but that is not a requirement. This are essentially beefed up reactions. I suggest a move type action (half speed, or teleport, or wings) and one or two attack options.
Both legendary resistance and legendary actions potentially increase CR. The CR increase for legendary resistance is in the DMG. For legendary actions, just figure out how it affects the DPR and the rest is in the DMG. That beings said, the 1-2 CR seems correct (assuming you didn't change anything else).