How do I save my custom campaign if I'm not ready to send it?

Custom campaigns do not auto-save, so you will need to click "Save Draft" in the upper right-hand corner of the Campaign Designer. 

To find your saved draft, click on "My Drafts" in the Campaigns tab. 

Then, click on the title of the campaign to re-open it. 

Hovering over the campaign also gives you the options to view a preview of the campaign, to replicate the campaign, or to delete the campaign draft.