In the Tumblr Editor, open your Account Settings and select Edit Appearance.
On the next page, select Edit Theme.
Click on the Edit HTML link.
Paste this exact script into your Tumblr HTML file. It's best to add it between the Head tags. Update, then save.
Create Text Post
Return to your Tumblr Admin page, and create a new Text post.
Paste this shortcode into the post and save. Expand the post to see your new Button plugin. Click on the Settings Icon to launch the POWr Editor.
Edit your Button on Tumblr:
Go to your live Tumblr website and click the settings gear in the upper right corner of the Button Plugin.