I have been very lucky to review for A.O.Com twice, and I'm often asked questions about how you go about becoming a reviewer and where to send it to so I will share my experience here.
The application process itself is very straightforward. All you have to do is an honest review of an appliance you already own, for example, your Microwave, Cooker, Fridge, Coffee Machine or Vacuum Cleaner. Make sure you film it in Landscape and mention any pros and cons about the product. Film it in use (you don't have to be in it), but make sure you are clear and descriptive. Ideally it should be between 2-8 mins long.
Once your done you need to upload it to Youtube and then take the link ready for your application submission on the A.O FB page. Unfortunately at the time of writing the post the "Apply to become a Reviewer" is no longer on their page but you can simply ask them if they need any more reviewers and you can share your video of them.
The next stage is normally a message from their staff saying that someone will be in touch if you're successful. Fingers crossed for your all. If you are lucky enough to get an email you will normally have the pick of some products they want reviews on. Once you've let them know and arranged delivery you have 7 days to submit your review and upload the video of your product.
The 2 products I've reviewed are here if you need some tips.