Login to your account. Click on the Setting icon (the one next to the Power icon for logging out). On your left hand side, you will see a vertical menu showing Account, Dashboard, Email, Apps and the last row, your blog name. The last row is what you should click, so click on your blog name. After clicking, you will notice the middle part of the page has a section called Theme with a "Customize" button. Click that button.
You will now see a whole new page. There is an "Edit HTML" button. Click on that.
This will open up a dark grey area showing the HTML code. Scroll down and look for the tag. Ensure that the body tag has an Angle Bracket before it. You can use the browser search function by holding down Ctrl + F (for Windows) or Command + F (for Mac) and type "".
Paste the code you obtained from us just below the tag as shown in the picture HERE.
Click Update preview when you are done. Then click on the "Appearance" button on the top left which will take you back and allows you to save. Press the SAVE button, followed by the Close button. Visit your page to see the big change!