During the Warring States period, the Qin have had the largest force for a couple hundred years since the fall of Zhou. The other six states were Qi, Chu, Wei, Yan, Han and Zhao which would later combine forces to rebel against Qin but didn't succeed. The plan is rather simple. Between the Qin and Yan is Chao, Wei, Han and Chu. Wei is located between Zhao and Han, while Han is located between Wei and Chu. So what Qin did was to ally with Yan and Qi to hold the four other states in between their alliance. By holding down Wei and Chu, the four states were not able to communicate and form allies with each other, since they will always be one state away from each other.

By conquering Han, Qin is able to isolate Chu from the rest of the states. When Zhao launched an attack on Yan, Qin attacked from the back and took over Zhao, inevitably Yan followed. At this point there is still only only Wei, Chu and Qi left. Wei is the next to be conquered because it's located right in between Qin Han and Zhao. Chu followed, and finally Qi too.

Qin unified China by allying the states from far away while attacking neighboring states. If anything were to happen, the allying state from far away could always attack from the back; conquering the enemy state while protecting an ally at the same time. The difference is Qin took the advantage and used it to also conquer the allying states.

After conquering all the states, Ying Zheng declared himself the first emperor of Qin.

Sorry that I skipped all the details, since you are asking for how Qin unified China, the short answer is in bold.

Sorry, no homework. It says in our rules: Our users aren't here to do your homework for you, but they might be willing to help. Remember: AskHistorians helps those who help themselves. Don't just give us your essay/assignment topic and ask us for ideas. Do some research of your own, then come to us with questions about what you've learned. This is explained further in this [META] thread. You can also consider asking the helpful people at /r/HomeworkHelp.

This is fantastic. Am writing an 8000 word dissertation so I would've it if you could give some mOre details and sources. Currently am thinking on focusing on shangyangs legalist ideas and the Qins ideological reform to make them into a militarised state, however the allying aspect hadn't really considered