Before begin, these few things are needed.
1. Text editor;
2. Site icons, I used fb & blogger in size 32x32 and looks just nice;
3. Site address, eg: www.facebook/yourUserName (click here more info);
4. Simple signature that still in use. (Will be replace after this) Click here if haven't got any.
Next, we shall begin typing your desire information using text editor including to drag in those sharing site icons you have prepare earlier.
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Next inserting those link info (sharing site web address) to those icons individually by doing so, highlight the icon then go to Format from menu bar, select Text then Link then paste the web address.
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Once done, hightlight the entire info that just created, copy them to clipboard then goes to Mac Mail, open up Preferences goes to Signature and paste it here or overwrite the current one)
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Done, hope it helps you.