a. MacOS: In the terminal, pass the --auto-open-devtools-for-tabs flag to while launching Chrome.

/Applications/Google\ Chrome.app/Contents/MacOS/Google\ Chrome --auto-open-devtools-for-tabs

b. Windows: Right-click the Chrome shortcut > Properties and add the flag at the end of the Target line. Alternatively, run the following from your command-line,

"C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" -auto-open-devtools-for-tabs