Need songs mastered

I typically get songs mixed and mastered by the same person but I'm trying to up my game and get it done separately, seeing this is my first time gettin it done separately what do you need in order to master just the track? and could you let me know the reason why it's better to do separately, I have been told it is but wondering why. Thank you - Anthony

Additional info:
The song is mixed and ready to be mastered. I'm interested in working with a very experienced mastering engineer. I don't have a hard deadline for this. I'm looking to master 3-5 songs. I'm an indie artist.