Spark — 6 steps, ~2 minutes

Copy button included below

1
Open Spark and choose Spark → Settings from the menu bar.
2
Click the Composer tab, then click Signatures.
3
Click "Create Signature" in the top right and give it a name.
4
Click inside the editing box and paste your signature.
Make your signature first →
5
Click Save.
6
Back on the Signatures screen, find "Default Signature" and choose this signature for each account you want it to appear on.

Good to know

Spark assigns signatures per account rather than one default for the whole app, so repeat the last step for every address you send from.
Paste your signature directly into the editing box — Spark's rich-text editor keeps the formatting, links, and images from a normal paste.
If a paste ever loses formatting, click "HTML" in the bottom-right corner of the editor and paste your signature's HTML source there instead.