To create an issue from a post, go to the post and click the issues icon:
After you have clicked on the Issues icon, you will be redirected to a new window with a "New Issue" 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 due date. You will notice you can also assign this issue to someone or take it on yourself to solve it. If you do not assign an issue, it would be automatically posted to the Issue Board (more details on the Issue Board see here). After you have filled in the information, click the blue "Add" box. Once you have clicked "Add" your post will be converted into an issue.