Login to your Salesforce account and create a new Letterhead. To do this, select the Settings icon from the top right menu, then select Setup Home.
From the menu on the left side, underneath Administration select Email then Letterheads. You may see a screen that says Understanding Letterheads, click Next on the right-hand side.
Create a new letterhead. Give it a label and a name and make sure to tick the Available for use checkbox.
Edit all of the available options including the Header, Top Line, Middle Line, Footer and Bottom Line and make each one white by typing #FFFFFF for the background color in the boxes available. Set the Height (in pixels) of the Header and Footer to zero. This is important as you don't want any extra lines or colors on your nice email signature! Now save your letterhead.
Now that you have a Letterhead you can build your Email Template. From the left menu click on Email Templates. Select My Personal Email Templates from the dropbox box, then click New Template to create a new one.
Choose HTML (using Letterhead) from the options.
Choose the "Personal Email Templates Folder", tick the Available for Use checkbox, give your template a new name and unique name, make sure to select the Letterhead that we just created and under Email Layout and select the Formal Letter option. Finally, select Unicode (UTF-8) as the Encoding option, this has the widest compatibility of all of the options and is what our email signatures use. See image below for exact requirements.
Switch out of Salesforce and open you email signature in your web browser. Once open, press Ctrl + A to select all, then Ctrl + C to copy the signature.
Now go back to Salesforce. You will see a subject field, in Lightning Force UI you're required to add a subject for your Email Template. Add whatever you like, but just remember to change your subject before sending your emails. Once you have added your subject, paste your email signature using Ctrl + V in the middle box as shown below.
Once you have correctly pasted your email signature and selected next, you should see text-only version. Select Copy text from HTML version and change any text if you wish.
Your email signature has now been created as a letterhead, and can be inserted when sending emails from Salesforce.
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