Before you begin installing the signature, you will need to see the hidden files and file extensions.

Go to your File Explorer (Windows key + E)
Select your hard drive (C:/ for most users)
Click on the Organize drop-down box
Click on the Folder and Search options tab
In the folder options, select the View tab, and make sure Show hidden files is selected, and uncheck Hid extensions for known file types
Now that this is done, we can get working on installing your signature.

Open File Explorer (Windows Key + E)
Go to Your Username/AppData/Roaming/Microsoft/Signatures/<.li>
Paste your .HTML signature in the Signatures file.
Rename the file by changing the extension from .HTML to .HTM
Open Outlook 2013 and go to FILE
Go to Options
Click on the Mail Tab and then on the Signatures button
You should see your signature in the list of Select signatures to edit box. The name of your signature will be your file’s name. Also, in the Edit signature box, your signature may not display properly. Do not worry, this is perfectly normal. Do not make any alterations. Make sure that you choose your signature in the New Message and Replies/Forwards
CONGRATULATIONS! Your signature is now installed!
– See more at: http://myhtmlsignature.com/installing-your-email-signature-in-outlook-2013/#sthash.Qj5eXA4I.dpuf