"He sticks them onto his bedroom walls" works a little better. Without more information I can't suggest anything more specific though. Keep up your studies!

No, you should NOT say "He sticks them onto his bedroom's walls." It should be "He sticks them onto his bedroom walls." Here, bedroom is used as an ADJECTIVE - it modifies the noun "walls". You can use words like bedroom, kitchen to describe other nouns. See the examples below: "He purchased a brand new kitchen sink." "I looked out my bedroom window." "I think the living room walls need repainting." "I am tired of looking at the same old kitchen wallpaper" "The courthouse ceiling is magnificent!"

Just "bedroom" not "bedroom's"

Thank you all! This certainly has been clarified. No more bathroom's towels! Haha

The bedroom doesn't own the walls (without the walls, there's no bedroom!).. so no, 'the bedroom's walls' is not right.