Turn off my phone completely. Put it in a desk drawer. Clean my desk completely. Flip my notebook to a new page. Close all chat/email apps. Open my IDE. Write down the most important thing to do today (check Jira if I'm unsure). Open Spotify and play a lyricless music playlist (jazz or metal). Work.
So far, this is the ONLY way I've been able to reliably get in the zone. It's crucial that I avoid Slack, email, or the web until well into the afternoon. Sometimes I feel mentally "fragile" for struggling to multitask or return to work after being interrupted, but if it works, it works. I'm lucky to be part of a team that isn't too keen on frequent meetings and doesn't expect instant Slack responses.