eM Client — 7 steps, ~3 minutes

Copy button included below

1
Open eM Client and click Menu in the top-left corner, then choose Settings. (On a Mac, the same items live in the menu bar.)
2
Go to Mail → Templates and Signatures.
3
Click the "Signatures" button to open the signature manager.
4
Click New and give your signature a name.
5
Click into the body area and paste your signature.
Make your signature first →
6
Save the signature and close the editor.
7
Back on the Templates and Signatures screen, choose the account this signature is for, then pick it for new emails and, if you'd like, replies and forwards.

Good to know

Paste the signature itself rather than its HTML source — eM Client's editor keeps formatting, links, and images from a normal paste.
Signatures live in one shared list, but each account is assigned its own — repeat the last step for every address you send from.
eM Client can use a different signature for new emails, replies, and forwards on the same account; the three dropdowns on the Templates and Signatures screen control that.