colonization started when European explorers came to Africa.
They came to explore Africa and look at the routes and different terrian and due to this they were able to know the different path, roads that connect Africa.

2. The Christian missionaries where also used as agent to colonize Africa
3.The European trademen were also used as agent of British colonization

And after this, the British colonial masters started trading with the people of Africa especially Nigerians in the legitimate trade and they started signing treaties with Africa chiefs and finally the Final destruction happened when in 1851 Lagos was defeated and king dosumu was killed and by 1861 Lagos was annexed and it became a colony on its own.
Then after that Benin kingdom and other northern empires were defeated and was brought under the control of the British.

The British was able to achieve this defeat due to their gun boat and maxim gun.
Thank you