I assume "Return to Mewni" is referring to Star and Moon returning to the Kingdom after Ludo invades and imprisons everyone.
Just working on the basic idea of narrative structure, that seems like the order things have to be in.
"Book Be Gone" is most likely the setup episode, with its less specific title compared to all the other very specific titles. The summary of the movie says something about "magic disappearing throughout the kingdom" which more likely actually is talking about Ludo/Toffee draining the Magic High Commission and the Book of Spells. Ludo wakes up with newfound power, Star and Moon set off to res the MHC at the temple, and we see Star spying on Marco being sad before/early into this quest.
"Marco and the King" obviously deals with Marco returning to Mewni and River celebrating this. Star and Moon must have already left. This must happen before shit really hits the fan, both for reasons of tone with that party scene and because we know they get locked up later after Ludo invades.
"Puddle Defender" involves Star and Moon meeting Buff Frog while they journey through the Forest of Certain Death.
"King Ludo" returns to Marco's plotline as Ludo invades while Star and Moon are gone.
"Return to Mewni" presumably involves Star and Moon returning from the temple to face Toffee.
"Moon the Undaunted" and "Toffee" both sound like extremely plot-heavy episodes and will most likely detail the fight between Star & Moon and Ludo/Toffee, with heavy flashbacks between scenes.
"Moon the Undaunted" might even be just one big flashback episode, and "Toffee" is obviously the half-hour conclusion.