Click on Manage metadata and select Tag standards. You will then see a list of all the available metadata fields (called tags). From here you can either select an existing standard by clicking on Choose tag standard and selecting the one you want from the dropdown menu, or you can define your own standard. You can do this by simply selecting the tags you wish to include, and move them one by one over to the window on the right hand side.
Once you have moved all the tags you wish to include over to the right hand side, remember to click on Save as new tag standard.
Give your new standard a name and a description and click on save.
The new standard will now be available when you explore your metadata.