Just talk to him.

I think it would depend on how close you would like to be of friends. If you just wish to be able to talk to them well and exchange ideas, then it should be relatively easy (depending on their personality in comparison to yours). Perhaps just start small and then work on. However, if you want to be more-active friends, you should first be more familiar - if you are not already - with them. Also, some teachers dislike being friends with their pupils for certain reasons. If you think they would be okay with doing activities outside of teaching you, maybe you can suggest some sort of trip to an educational environment (maybe there is an English hub somewhere nearby he can take you to and explain about or a museum or something. :P). Then work from there. Sorry if that's not entirely helpful. I hope it assisted you in some way. :D