Apple Mail — 7 steps, ~3 minutes

Copy button included below

1
Open Mail and choose Mail → Settings (Preferences on older macOS versions).
2
Click the Signatures tab.
3
In the left column, select the email account this signature is for.
4
Click the + button below the middle column to add a new signature, and name it.
5
Select all the placeholder text in the signature box on the right and delete it.
6
Paste your signature into the now-empty box.
Make your signature first →
7
Uncheck "Always match my default message font," below the signature box, so your signature keeps its own fonts and colors.

Good to know

"Always match my default message font" is checked by default. Leave it checked and Mail strips your signature's fonts, sizes, and colors to match your regular message font — uncheck it before or right after pasting to keep your design.
Images can look blank inside the Settings preview — that's normal. Compose a real email and they appear.
Still seeing plain text? Toggle "Place signature above quoted text" off and on, then paste again.
If you created the signature under "All Signatures" instead of a specific account, drag it onto the account in the left column, or it won't show up when you're composing.