Why?

I looked up "British Accent" on a search engine and it pulled up Wiki pages, Youtube videos, and other websites about learning how to speak in a British accent. I'm sure some of the Youtube videos will have directions (i.e. zooming in on the mouth so you can mimic) so try some of those out for sure. You could also try asking a major bookstore in your town if they can order in a phonetic guidebook for you, that is sure to have useful information on pronouncing vowels and certain words. Otherwise you could try looking at an online bookstore for a wider range. One page I found warns that there are many various British accents, so you should try researching them and choosing a specific accent. This is so you don't start speaking in a Scottish, London, Cockney, Liverpool accent in the one sentence. Choose your region of British accent and research just that one.

Hello, Yes practice with native speakers and listen to lots of British English broadcasts , watch movies in British English , lots of listening will help you for sure ;)

if you have enough money you should go to British another best way,you can find a native speaker to practice it good luck