Go to your Account Settings and click Edit Appearance. Then click Edit Theme.
Underneath Custom Theme, click Edit HTML.
Copy and Paste Script
Paste the POWr script into the HTML file between the Head tags. Then update and save.
New Text Post
Back inside the Tumblr editor, add a new Text Element.
Copy and Paste Shortcode
Paste the POWr shortcode into the text post, then save You'll see the new POWr Popup plugin when you expand the post. You can edit POWr Popup by clicking the POWr Settings Icon in the corner.
To edit your Popup on Tumblr:
Visit the live Tumblr web page and click the settings gear in the upper right corner of your Popup Plugin.