To convert a post to an idea, choose the desired post and click the yellow lightbulb:
After you have clicked on the lightbulb, you will be redirected to a new window with a "New Idea" box:
Here you will notice that the original text from the post you chose to convert has been added to "description". Simply choose a new title and category, then click the blue "Add" box. Once you have clicked "Add" your post will be automatically converted into an idea.