Read the script and note all of the different settings in each scene. Develop your decor based on scene changes and whether props are needed for a particular scene. While you will have a base set to work with, scene changes are accomplished by having stagehands introduce new stage props and decorations to make it appear like a new and different place.
Typical scenes in "Les Miserables" occur in the ABC Cafe, the Thenardiers' inn in Montfermeil, and Bagne prison in Toulon.
Develop your base set. The base set is the general background with stationary props where each smaller scene takes place. Since "Les Mis" takes place during a period of French turmoil, you may want to combine old architecture with a damaged aesthetic to reflect the violence and tumult of the era.
The base set will be an outside location, with each movable set being inside a particular building. Because of this, your base set should reflect the outsides of buildings and landmarks in Paris.
Create scene change props. These props are taken onstage and offstage to reflect a shift in location for each scene. Scene change props specific to "Les Miserables" would be the table and mugs for the ABC Cafe, prison bars for Bagne prison, and the reception desk of the Montfermeil Inn. These props help contextualize the action for audience members and can be easily moved around on stage during scene changes.
Decorate the venue with appropriate thematic props. Your performance space should reflect the ideals of the musical as much as the stage. You can hang chains around the auditorium to reflect Jean Valjean's captivity and struggle for redemption or use guillotine props.