Here is a video, made by the RYA, explaining how to tack reach to reach and then close reach to close reach and featuring Olympic Medalist, Shirley Robertson.
Note that following the tack Shirley adopts a "frying pan" grip, before moving to a dagger grip by bringing the extension over her shoulder. This is because she is sailing a Pico and the tiller extension is too long for her to bring it under her arm. Mirror extensions are shorter, so it's possible to use the approved RYA method for those at Stage 2 by moving the mainsheet hand back to the tiller and grip it with fingers down (i.e. dagger grip), then, after releasing the mainsheet, bring the tiller extension under your arm so it is in front of your body (as per the RYA National Sailing Scheme Instructors Handbook G14 page 59). This works well on a Mirror, so long as you sit in the right place (i.e. well forward, near the thwart).