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Settings and Options

Edit Signature
type your words in the box.

You can use the features... like Centre, Bold, Change Text Size, add Colour etc..
You can run the cursor over the symbols and it'll tell you what they are.

From memory, I think you need to highlight the text before you use the feature.
e.g. if you want to Centre, highlight the words and then click on Centre

You then need to press "Save Signature" and that's further down the page