Clone into phpBB/ext/nicofuma/webprofiler:
git clone https://github.com/Nicofuma/WebProfilerExtension.git phpBB/ext/nicofuma/webprofiler
Go to "ACP" > "Customise" > "Extensions" and enable the "Web Profiler Extension" extension.
Navigate to app.php/_profiler/ or use the link in the toolbar to consult the reports.