So you've picked an open-source license from choosealicense.com, huh? FANTASTIC!
Now comes the fun part: adding that license to all your public repositories. This website will do just that. It'll give you a list of your public repositories that don't have a license file. You'll pick which license you want, and it'll add the new license file to your repos. SIMPLE!
This site is not owned by or affiliated with GitHub. But I'm using the API to add each new license file. You'll be asked to authenticate this app for your public repositories on the next page.