Navigate to your downloads folder and cut or copy the HTM file.
Open your outlook signatures folder by going to %APPDATA%/Microsoft/Signatures/ in your file explorer. Alternatley you can hold the Windows key + 'R' and paste %APPDATA%/Microsoft/Signatures/ into the window that appears in the bottom left of your screen.
Now just paste the "HTM" file into the signatures folder. If all goes well, it will now appear in your signatures list.