Just read it right to left and let the particles tell you how to build the sentence!
친구입니다 - Is a friend.
제 친구입니다 - Is my friend. 제 describes 친구.
배우가 제 친구입니다 - The actor is my friend. ~가 tells us the subject.
있는 배우가 제 친구입니다 - The actor that exists is my friend. Any verb phrase that ends in ~는 describes the noun that immediately follows.
가운데에 있는 배우가 제 친구입니다 - The actor that is in the middle is my friend. ~에 specifies where the actor is "existing".
Prepositions go after the noun when describing where they are. 식탁 밑에 있어, 상자 안에 있어, 침대 위에 있어 (it's under the table, it's in the box, it's on the bed). When trying to distinguish a noun, like in this sentence, "The actor in the middle," it follows the same rule as any other like clause, so you move the preposition and the verb before the noun.
I'm not sure when a preposition and an adjective are related. Do you have an example in mind?
Sorry I made the mistake of thinking that 가운데에 was a preposition preceding the adjective 있는 but now I see that the two words together are 1 adjective (I think?)
I guess I am struggling with how sentence structure goes asides from the simple SOV etc. such as what order adjectives go in when there are multiples (if it matters as it does in english) where adverbs go, and which parts of a sentence can be moved around without changing the meaning. If you've got a good resource for me to look at I'd love if you could share that!