When looking for someone, know what kind of music exactly you are looking for and be open to the not so obvious as well. The music industry is full of talented musicians who would be honored to take their craft to the big screen.
Take the music that you like first and play it with the film, see if it works. If not try something else. Once you have a basis for what works well, then you can find a composer to work with that.
Know your budget. Have money set aside at the beginning for a composer or whomever you are working with for the music. With this being one of the 7 best ways people make money off music you can’t just think of them as a last minute thing and pay them next to nothing.
They are working hard just as the actors are for the part so ensuring that they are getting a fair share, and are actually getting paid, is important as well.