Imagine that this person has a personality & set of speech patterns & habitual behaviors which can be related to her/him specifically and that are rich with repetitive non-sequiturs, references & inside jokes.
Attempt to mimic these attributes as closely as possible, integrating them into your daily personality & behavior. Do so without trying to understand them, to "get the jokes", or to create any other kind of bridge of translatability between yourself & the personality you are assuming. Be as thorough as possible.
Refrain from learning the name of the person you are imitating, as this prevents rigorous imitation. At all times consider the speech patterns, jokes, personal references, emotional states, etc. that you mimic to be your own. This makes complete projection possible.
Invent a means to explain the accumulated attributes to people with whom you are close & who notice the changes in your behavior & use of language.