In Return of the Jedi Luke displays a greater affinity with the Force and increased skill with the lightsaber, not to mention having learned to build his own. Where did Luke hone his skills between Empire Strikes Back and Return of the Jedi? Did he return to Dagobah and train with Yoda, or did he go somewhere else?
The film canon doesn't make it clear exactly how much time elapses between the three movies - but other canon indicates that there is about 4-5 years between the destruction of the two Death Stars, and about 3 years between the Battle of Yavin (Death Star I) and the Battle of Hoth. This means that there is about a year between the end of Empire and the beginning of Jedi.
It seems that Luke and the others were busy with the rebellion over this intervening time. This would have allowed Luke to meet challenges and grow in his powers between the two movies, without the need to return to Dagobah.
Seems to me that if Luke got further training from anyone, it would have been from Obi-Wan's ghost. Luke's attempted rescue of Han has (prequel) Obi-Wan's style written all over it; it is cocky, ballsy, and incredibly cavalier, employing mind tricks and bravado to get close to Jabba (and I wouldn't put it past Obi-Wan to Force Choke a couple of grunts to unconsciousness if it was more convenient than trying to talk his way past them). Then Luke confidently walks right up to Jabba, and demands that the Hutt cough up Captain Solo and the Wookiee (Yoda would have known that Hutts were immune to Jedi mind tricks).
Luke has layers of contingency plans in place, with Leia, Lando, Chewie, and the droids right where he needs them to be (think how things would have gone down if Chewbacca hadn't been on the skiff with Han and Luke), and even when things seem to be going Jabba's way, Luke maintains that cocky demeanor, giving Jabba one last chance to reconsider, or be destroyed. Then he pulls off a spectacular escape, and blows up Jabba's sail barge for good measure.
Think how Yoda (in his prime) would go about rescuing Han. He would probably go in alone, using stealth, subterfuge, and distraction to sneak into the palace. Then he could defrost Solo without being discovered himself, and once Jabba has Han tossed into a cell, Yoda could easily infiltrate the jails, spring Solo, and sneak back out again, using violence only as a last resort.
The important distinction is that, had he been so inclined, Yoda could probably have blitzed into the palace in the night with extreme prejudice, like a little green blur of death, striking down every single son of a bitch in the building, but that's just not his way.
On Tatooine, mostly.
In the Shadows of the Empire project, a multimedia story meant to act as an interquel between Empire and Jedi that included novels, comics, musical pieces and video games, we learn that Luke returns to Tatooine with Leia, where they reside in Ben Kenobi's hut.
On Tatooine, Luke trains his acrobatic skills on the acrobat string of a circus tent, which he gained access to by mind tricking the guards at the entrance. During his stay on the planet, he also meditates on his training with Yoda and his encounter with Vader. This makes him grow spiritually, until he believes he can attempt creating his own lightsaber, using the instructions left to him by Obi-Wan in the form of a book filled with Jedi Knowledge. Not possessing any force imbued crystals, Luke eventually has to grow one in a blast furnace, guiding its formation using the force (a process very similar to the one used by the Sith, in fact). I can't find sources on it, but there used to be a fan theory that Luke canibalized parts from Qui Gon Jin's lightsaber (which Obi-Wan kept after his master's death), but this is not supported by the material in Shadows of the Empire, which describes Luke gathering the needed parts in Mos Eisley.
While constructing his lightsaber, Luke takes a break and rejoins Rogue Squadron for the Battle of Gall. This also has the side effect of allowing Leia and him to get back on Boba Fett's trail, hoping to find Han. Tracking him to Tatooine, Luke goes back to finish his lightsaber.
After completing his lightsaber, Luke's time is spent defeating biker gangs working for Jabba, capturing secret supercomputers with the help of the Bothan Spynet, being captured by bounty hunters and defeating Xizor's plans. These adventures allow him to consolidate his training and honing his lightsaber and Force skills in real combat situations. Experience can go a long way, and this could explain the great improvements to his confidence and skills between Empire and Jedi.
TL;DR Luke goes back to Tatooine and lives a few months with Leia in Ben Kenobi's hut, where he meditates on his training with Yoda. He also learns a lot about the Jedi (and how to construct a lightsaber) from a book filled with instructions left to him by Obi-Wan. He then hones his newfound skills on the hunt for Boba Fett and defeating crime lords.
Luke could not have learned from Obi-Wan's Ghost or Yoda between the films because he spoke to BOTH of them in RotJ about the revelation that Vader was his father. He wouldn't have just finally got around to talking to them about something like that in RotJ unless that was his first encounter with them since being told about it.
He practiced what he had learned and studied Obi-Wan's notes in Obi-Wan's house on Tatooine. He was able to learn much, including how to build a lightsaber. The rest he learned through experience.
I always thought it was strongly implied that Luke returned to Yoda for further training, because when he returns again after saving Han from Jabba, Yoda tells him that he requires no further training. Considering how crucial it seemed that Luke learn from a Jedi master, I can't imagine he would somehow teach himself after leaving in ESB, when both Yoda and Obi-wan are practically screaming that he's not ready. In the Star Wars radio show (RotJ was produced for that series much later than the first two parts, in 1996) there is a scene in which Luke constructs his lightsaber by following the instructions found in Ben Kenobi's house on Tatooine.
Read Shadows of the Empire. Luke completed his training himself. With the aid of written materials in Obi Wan's hut on Tatooine. Luke is a Force savant. His affinity with the Force and abbreviated training with the best Jedi Master ever elevated him to a Jedi Knight. Remember Mastery of the Force often occurs via experience and spiritual growth. The truth set Luke free. He met his Father head on and overcame his doubt and reluctance. He unlearned what he has learned. :)
Being non-canonical, Shadows is out and we can note that Luke uses dark force powers in his assault.
We know that Luke, like Anakin, may well be capable of force rage, as seen in his final battle. But really Luke was not a shadow of Anakin at his age, and I believe Palpatine used his known powers to enhance Luke enough and weaken Vader enough; Just as he did with Dooku. But Luke was not a little egocentric bitch and resisted.
Ok. Back to where he learned them: with Holocrons and Maul alive (and Ashoka maybe being alive as well); any one or more of these can be the answer or even training with non-aligned force users.
In Shadow and Beyond Luke was going out of his way to learn, all while secretly always remaining true to the greater good - or just to hook up with Mara Jade... I don't know...
With all that undone and Rebels including Luke as well as the comics and games it will all fill back in through Disney's eye.
Read Shadows of the Empire in the novel form, comic form, and play the video game on either the N64 or PC. That will explain the story in between Empire Strikes back and Return of the Jedi. Canon or not, this is the story in my opinion. And it's a one-year gap between Episode V and VI, not 3 years like people think it is for some reason.