you're not a jedi until you create your first lightsaber. Vader in RotJ "I see you have constructed a new lightsaber. Your training is now complete."
Thats what he was doing. He went back to obi wan's hut on tatooine and created new lightsaber. I always thought he found Qui-Gons old crystal, but everyone else seems to say that he created a synth crystal. I have read most of the novels, but not the comics so I am not entirely sure abou tthe crystal.
But thats what he was doing. Once you create your saber using only the force....your training is complete.
He created the crystal in a furnace using instructions he found in Obi Wan's hut, that Jawas and Sand People thought was haunted.
This is actually stated in Shadows of the Empire, Luke was also afraid his lightsaber would blow up in his face when he first turned it on.