Only Growth Masters have the ability to convert an idea into a test.
When you've determined an idea is worth testing, go to the "Ideas" section and locate the idea you want to test. Mouse over the idea and select the "Test" option.
You will then be asked to enter a hypothesis. You will not have the option to test an idea without a hypothesis, so make sure it's a good one. Once you do that, it will move into to "Up Next" lane within the "Test" section.