You must follow the necessary things for better use of this plugin-

App Controller (yourapp/src/Controller/AppController.php) Download App Controller
This controller must use 'Usermgmt.UserAuth', 'Usermgmt.Image', 'Form' helpers.
This controller must use 'Flash', 'Auth', 'Usermgmt.UserAuth' components.
This controller must have 'paginate' override function.
Plugin already has Security feature. If you want use security on rest of the pages of site you should use 'Security' and 'Csrf' component in this controller.

CakePHP Auth Component
This plugin is uses CakePHP 3.x built in Auth component.
You can change Auth component configuration in UserAuth component beforeFilter function.

Bootstrap (yourapp/config/bootstrap.php)
This file is used for loading plugins.
Please load User Management Plugin in this file
Plugin::load('Usermgmt', ['autoload' => true, 'bootstrap' => true, 'routes' => true]);

Please note in above syntax 'autoload' => true, 'routes' => true, 'bootstrap' => true must present.

CSS and JS files
Your layout must include plugin CSS and JS file and other things. Download Default Layout
You must include plugin css, js and other setting in your default layout (yourapp/src/Template/Layout/default.ctp)
For more information refer README file

Library and Image Folder
By default profile photos folder name is umphotos (yourapp/webroot/library/umphotos)
This folder must have write permissions for e.g. 777 permissions
Also library folder (yourapp/webroot/library) should have write permissions for e.g. 777 permissions

Yahoo Authentication
If yahoo authentication is not working on your website please contact me at chetanvarshney@gmail.com