Hi,
Method 1
1. Download DebugKit from official repo as zip package:
https://github.com/cakephp/debug_kit2. Go to your QACMS admin panel; Extensions -> Install New Plugin
3. Use the "Upload Plugin Package" section and select the packaged downloaded at step 1
4. Check the "Activate after installation" option
5. Press "Upload Package" and you are done

Method 2
Use composer to install debug_kit,
1. composer require cakephp/debug_kit
2. After composer is done, go to you QACMS admin panel
3. You will see an alert indicating there some plugins awaitings for installation
4. Follow the GUI instructions and you are done
Regards!