Nazwa forum

Simple

You need to:
1) choose your city, where you have soldiers
2) go to view of the world
3) choose an island, where you are looking for a target
4) click on your target
5) in left menu you have "pillage".
6) wait for battle

Thats all

However, if you are asking how battles works... hmm... Who knows that? I havent even found in board informations, how battles go