"It's easy to get started: visit docs.google.com in a browser on a supported device, and select the document you want to edit. Then, when you're viewing it, press the Edit button to switch to the mobile editor," suggests Google.
I couldn't find the Edit button, but that's because the new features aren't yet available to everyone. Google promises that the mobile editor will be rolled out in the next few days and you won't have to buy an iPad or a Samsung Galaxy Tab to use it because it will also work on an iPhone or an Android phone that runs Froyo.
Unfortunately, the mobile editor will only work for documents created using the new document editor. It's difficult to understand why Google couldn't find a way to convert the documents created using the old editor to the new format.