Hello jellyju3u,

I will do my best to instruct you step by step.

1. First of all you will need an alliance. (If you dont have an alliance, send me a message and ill show you how to make one)
2. Go to the Conflict of nations menu and click on the RANKING tab.
3. It will direct you to the players ranking list. Right next to the Players tab you can see the Alliance tab, which shows you the Alliance ranking ladder.
4. Click on the alliance tab and type the name of the alliance you would like to challenge in the search bar.
5. After you find them click on their name and a small window will pop up. You will see the Challenge button underneath the alliance's picture.

I hope i was instructive enough.
Have fun Soldier.