Outlook (classic) — 8 steps, ~3 minutes

Copy button included below

1
Download your signature as an HTML file.
Make your signature first →
2
In Outlook, open a new email, then go to Message → Signature → Signatures…
3
In the "E-mail account" list, choose the account this signature is for.
4
Under "Select signature to edit," click New, name your signature, and click OK.
5
Recommended: open the file you downloaded in a browser, select all of it, copy it, then paste it into the "Edit signature" box.
6
Advanced: instead of pasting, close this dialog, copy the downloaded file into your Signatures folder (hold Ctrl and click the Signatures button to open it), then reopen Signatures to select it. This is handy if you're setting the same signature up on several machines.
7
Under "Choose default signature," set this signature for New messages and, if you'd like, Replies/forwards.
8
Click OK to save.

Good to know

Outlook won't add the signature to an email you already had open when you set it up — only to messages you start afterward. Add it to that one manually.
Classic Outlook sometimes turns pasted images into VML, which can make them show up as attachments instead of inline pictures. If that happens, delete the image and re-insert it with the Signatures dialog's own Picture button.
Placing the file in the Signatures folder yourself (step 6) is the reliable option for POP/IMAP accounts — automatic roaming only covers Microsoft 365 mailboxes, and only if your organization has turned it on.