The ultimate guide to building standings is “The Plan”:
Read through that material, and it will give you a lot of great info on the most efficient way to build up your standing.
Also, this file is incredibly helpful in your progress:
It is a bit dated, but still accurate and functional as far as I know (save a local copy for you to edit). This also has a section that will give you the proper sequence in which to look for agents for the COSMOS missions as well as notes on what you need, what you will do for them, locations, and standings required.