George Lucas wrote a treatment--as in he laid out the foundations for the story. His script was only around 200 pages (most of which was changed dramatically when Alan Dean Foster NOT Goerge Lucas, wrote the first Star Wars book (The Adventures of Luke Starkiller) for him for a payment of $5,000; which was released a year before the movie came out.
Lucas has admitted this only recently--and the newest editions of the book give the credit.
Lucas did this:
I want Rebels
I want Empire
I want Jedi
I want Civil War
I want a master etc etc etc
He gave vauge ideas on where he wanted the story to go... He did have a 200 page treatment which he gave to Alan Dean Foster when he wrote the first book. Alan changed major parts of the treatment for it was for lack of a better word--terrible.
Alan then released the book under George's name and Lucas took all the credit (till a few years ago).
On the matter of the movies--Lucas adapted what Alan wrote into a screenplay then directed episode 4--which was a disaster due to 2 things; Lucas is a terrible director; this is why episodes 5 and 6 were better than all other films he made--for all others he directed. The other thing was his god awful editing--if you want to see how bad his editing is just look at the dialogue scenes in the prequels, absolutely bare minimum no excitement nor entertainment, predictable and utter a bore to watch; luckily he was able to get his wife to move on into the editing on episode 4 and she did a fantastic job--she gave scenes that felt lifeless; life.
In regards to episode 5:
Star Wars Episode V: The Empire Strikes Back (also known as The Empire Strikes Back) is a 1980 American epic space opera film directed by Irvin Kershner, produced by Gary Kurtz, and written by Leigh Brackett and Lawrence Kasdan, with George Lucas writing the film's treatment and serving as executive producer. Of the six main Star Wars films, it was the second to be released and the fifth in terms of internal chronology.
George did not write it, the screen play was written by Leigh and Lawrence based on Lucas's vague treatment--the success to 5 is the director "Irvin" and the amazing themes and scenes he forced the studio to put into the movie.
Episode 6 was written by George and Leigh. Hence why it had Ewoks in it(just look at Jar'jar and you can see what I mean by that).
George wrote episodes 1,2,3 all on his own--no editors for the screenplay nor any proof-readers to fix continuity or coherence problems--what we saw was basically a first draft of each film--the novelizations were far better for 2 reasons: they weren't written by George--they didn't have the problems the movies had.
Hope this helps.