There are different ways:
1. Governor policy. But beware people of your religion assimlate faster. So better try to convert them first
2. Buildings like Theater or marketplace
3. Tech
4. Wonders
5 Roads
Doing so there is also a further strategy called deportation and concentration. You can turn all the people into slaves. Slaves assimilate faster: harsh treatment as governor policy helps to demote them into slaves faster in the first place. After this you just move most of these slaves into the towns with the buildings. There they assimilate faster .
You can also just overpopulate the towns . The remaining free people try to migrate away into other regions. In toher regions they are a minority.
