Set up the main drive
I put the tethered motors* back to back on a brick adapter so that I could connect them to a lego plate. Then I connected the lego plate to some technic frame pieces.
*These absolutely have to be the tethered motors so that we can make use of the 'Variable' setting so that they can go both forward and backward.
Set up gears
Using the lego frame you built around the motors, set up a vertical place to add gears, the first gear you will attach to the motormate will be a smaller one. We are going to gear down the motors so that the bot moves slow but it will be more robust to go over obstacles (the other legos laying around).
So Chain a few gears together having the smaller gears turn the larger gears.
Add some balance to your frame
Use something to connect some swiveling wheels to the front (and back if possible) These will keep the bot from tipping.
Add a frame piece for the front of your bot.
Connect the button bit to the center of the fork and put the light sensors on either side. Put together a frame for the front of your bot and connect it to the case holding the motors. Using another brick adapter piece or pieces connect the button and the light sensors to the front of your bot.
Add the arduino and connect it to your bots head
Connect the arduino using a brick adapter to the head region or wherever it fits just leave it in a good place you can connect it easily to upload its commands.
Set up your bits, and play around with your bot!
Tweak / Play
As you test your bot be sure to refine it, you may need to increase / decrease the sensitivity or play with the wheel positioning, or create a bumper for the button so that the bot doesn't get trapped in the corner. Maybe it's not a robot that likes the light but a bug bot that likes to hide in the dark spaces - reverse the light sensors to see what happens.