1. Tick the "Filter by folder" tickbox, which is when the "Root" folder drop down will appear.
2. Click the "Root" button, and the following pop up will appear:

Selecting the "Add Folder" button shown above will allow you to create and name your new folder. The other 2 buttons will allow you to edit and delete folders respectively.

Once you have created the new folder, you are able to add previously created workflows to it by ticking the tickbox next to the workflow title in the select screen, and using the dropdown arrow next to the "Root" button, select "move to folder". You will then be given the option to choose which folder to move it to.