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1. Right click on the src folder of your Flex Project and select New > Other choose Infinite Module from the Flash Builder folder
2. Give your widget a name and select finish. (If the wizard does not pick up the src folder automatically, select Browse and navigate to your projects src folder e.g. /projectname/src)

3. The wizard should automatically add 2 files to your project a sandbox to run your new widget in named InfiniteTestApplication.mxml and your widget which you named in the previous step (e.g. sampleWidget.mxml)
4. At this point you can run the example by right right clicking the InfiniteTestApplication.mxml selecting Run As > Web Application

5. Add your widgets logic into the sampleWidget.mxml file (or whatever you named your widget) and run sample queries by logging in and then searching on a term.

Alternative Method

As an alternative you can create a Flex project and paste our Sandbox Code and Widget Skeleton code into your project.