In this episode, we talk about tacking a centre main boat. In the example, I talk about taking a boat from a starboard tack to a port tack, close-hauled to close-hauled, 11 o'clock - through the wind at 12 o'clock - to 1 o'clock. The basic steps are:
Look behind you, checking for obstacles.
Communicate to your crew (Helm: Ready about. Crew: Ready).
Move the foot at the back of the boat over the strap in the middle to the far side. 
Push the tiller away from you, this will turn the boat towards the wind.
Duck under the boom to swap sides, looking forward and NOT letting go of the main sheet or tiller.
Seat on the port side as far forward as you can.
Straighten up the boat when you are at the 1 o'clock position.
Now we start the hand swap.
Slide your right hand along the mainsheet to the tiller and with your index and thumb looking like a trigger, grip the tiller extension and the mainsheet. Your right hand will now be holding both the tiller and the main sheet. Your left hand is free now to reach forward and grab the main sheet as it comes out of the block. Your left hand is now in full control of the main sheet. 
You can now release the main sheet from your right hand and flick the tiller extension over your shoulder and return to the normal sailing position.
This video may help if you need it: https://www.youtube.com/watch?v=ina817FglgA
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