A lot of people think that 'arranging' is just something that anybody can learn from a book. The only people who can learn it in that way are the ones who studied harmony and know how to harmonize a melody effectively. That's not a little theory.
Composition and arranging are the two areas about music where every musical skill needs to really well developed , even at a 'basic' but successful level. They require complete, and deep, musicianship. You can't just learn a little theory here and there and hope you are an arranger or a composer. And you can't learn it in a forum post. Sorry, but it's the truth. I hear a lot of 'arrangements' around and I can immediately tell when the 'arranger' is not ready to be an arranger, simply because 1) his knowledge of harmony is weak, and 2) he doesn't understand how to harmonize a melody well. If you aren't able to do these 2 things really well first, it's like going from moped to F1 all of a sudden.
I am not saying stop trying. I am just saying, you need to have done some really solid preliminary work first.