Please make sure that the installation guide you are following is the right one for your Mac OSX. There are many different guides for Apple Mail and each one is different, depending on which OSX you are using.
You will need to check which OSX you are using before installing your email signature and then follow the correct guide below:
If your email signature social media icons are stacking vertically, you are most likely using the Apple Safari browser which causes images to stack.
Please use only Chrome or Firefox to login to your Email Signature Rescue account and copy the signature from or to open the HTML file. This will fix the issue.