This userscript alters the parameters of embedded Twitch players to overwrite both the default autoplay setting and the intentional autoplay setting.
To execute the userscript in your browser, I recommend using Tampermonkey. Tampermonkey provides an environment to create, store and execute userscripts in your browser.
Once Tampermonkey is set up and ready, click this link to start the install process of the userscript. Tampermonkey recognizes the userscript and offers you to install it. Once installed there's nothing else to do.