Drop "h" at the beginning of words and omit the "g" at the end; "appy' instead of "happy," for instance, and "runnin" instead of "running." Substitute and "f" for a "th" as in "somefing" instead of "something." Speak quickly and try to imitate the Cockney's distinctive, slightly nasal twang.
Immerse yourself in films and TV programs that portray Cockney characters. Observe Michael Caine in "The Italian Job." Watch the BBC soap opera "Eastenders" and any of the Ealing comedies made between the 1940s and 1960s, such as "Kind Heart and Coronets" or "Ladykillers." Study popular comedy programs featuring Cockney characters, such as "Only Fools and Horses" or "Steptoe and Son."
Read up on Cockney rhyming slang. This probably originates from the costermongers (street produce vendors) who developed a way of speaking that was almost like a secret code. Rhyming slang has further developed and has become even more complex. An example of Cockney rhyming slang is to tell someone you are going for a "ruby". This originates from "Ruby Murray," and "Murrray" rhymes with curry. Another example is to say you are buying a "whistle." This comes from "whistle and flute." Flute rhymes with suit, so you mean you are buying a suit.