Dragon Age Keep—that's the web app that will allow you to recreate your past choices from previous Dragon Ages, and then import that world state into Inquisition—is now in open beta. You can sign in here with your Origin account (presuming you have an Origin account), and then use the resulting data in Inquisition when it arrives next month. The Keep itself takes the form of a sort of interactive tapestry, and can be toyed around with on anything with an HTML5-capable browser.
You're probably still wondering how it all works. Essentially it's a flashy questionnaire, with illustrated recap bits voiced by Dragon Age 2's Varric, that allow you to leap in at key decision points to divert the story in a direction of your choosing. You can't import save data directly, but if you've linked a BioWare account to your Origin one, the app will be able to read a certain amount of your past deeds automatically—though you can tinker about with the history afterwards as you see fit.
BioWare have provided their own explanation with the following trailer. Inquisition, meanwhile, is out November 21st.