Well, what you said was “That was a stupid movie I saw”. It’s a shade clunky, but not overtly wrong… I think. Feels a bit like two different sentences mooshed together – その映画はばか plus 映画を見た. If you wanted to say “that movie I saw was stupid”, you’d say その映画を見たのはばかです or その見た映画はばかです but both probably use grammar that’s a little above your level. =)

You can conjugate pre-nominal adjectives, but the overall meaning of the sentence is slightly different.

その映画はつまらない = “that movie is boring”
その映画はつまらなかった = “that movie was boring”

つまらない映画を見た = “I watched a boring movie” and it’s still boring.
つまらなかった映画を見た = “I watched a movie that was boring” but isn’t any more (perhaps because someone explained the complex plot, so now I enjoy it more).

I think Joel’s second sentence is most appropriate.

Descriptors in Japanese are always but before that which they describe. In English, adjectives come before the word they describe (ugly dog), but we also have ways of describing things later in the sentence. In “the movie that I saw”, the “that I saw” part is describing the movie, but comes after the word “movie”. In Japanese these types of descriptors come before the word they describe, just like adjectives.

So the sentence “The movie I saw was stupid” rearranged to “Japanese order” would be “The that I saw movie was stupid”. Or みた　えいが　は　バカ.