Creating a custom dashboard widget

Here is the default Dashboard panel seen in WordPress 5.9.

Remove default Dashboard Widgets

Removing all the dashboard widgets.

function wporg_remove_all_dashboard_metaboxes() {
    // Remove Welcome panel
    remove_action( 'welcome_panel', 'wp_welcome_panel' );
    // Remove the rest of the dashboard widgets
    remove_meta_box( 'dashboard_primary', 'dashboard', 'side' );
    remove_meta_box( 'dashboard_quick_press', 'dashboard', 'side' );
    remove_meta_box( 'health_check_status', 'dashboard', 'normal' );
    remove_meta_box( 'dashboard_right_now', 'dashboard', 'normal' );
    remove_meta_box( 'dashboard_activity', 'dashboard', 'normal');
    remove_meta_box( 'dashboard_site_health', 'dashboard', 'normal');
}
add_action( 'wp_dashboard_setup', 'wporg_remove_all_dashboard_metaboxes'  

The result of removing all the Dashboard widgets.

Dashboard Widgets – Developer documentation WordPress.

Add a new Dashboard widget

Creating a custom dashboard widget.

/* Dashboard widgets */

add_action( 'wp_dashboard_setup', 'dashboard_add_widget_gethelp' );
function dashboard_add_widget_gethelp() {
	wp_add_dashboard_widget( 'dw_dashboard_widget_help', __( 'Get help to manage your web site', 'dw' ), 'dw_dashboard_widget_help_handler' );
}

function dw_dashboard_widget_help_handler() {
	_e( '<p style="color: blue;font-size: 18px;"><strong>
Welcome to the backend of your WordPress web site!</strong></p> 
<p>Some helpful advice is located here: <a href="https://www.easywebdesigntutorials.com/" target="_blank" style="text-decoration: underline; font-weight:strong;">Link to the help page</a> </p>
<p>Contact <a href="mailto:">Paal Joachim</a> when questions arise.</p>', 'dw' );
}

The result of adding / creating a new dashboard widget.

Regarding code:
echo ‘some information to display’.
_e( ‘This is some text.’, ‘dw’ ); (e for echo also adds the opportunity to translate the text inside the single quotes ”.

https://stackoverflow.com/questions/5775135/what-does-the-wordpress-e-function-do
https://stackoverflow.com/questions/18252433/how-can-i-style-a-php-echo-text

Creating a custom Dashboard Welcome Panel

Removing the default Welcome Panel. Adding in some text and a link to a Youtube video.

/* Add a custom Welcome Dashboard Panel */

function my_welcome_panel() {
 ?> 
<div class="welcome-panel.welcomepanel::before" style="display: none;"></div>
 <div class="getting-started">
 <h3 align=center style="padding-top: 20px;"><?php _e('Taking your first steps with WordPress' ); ?></h3>
 
 <p align=center><iframe width="640" height="360" src="https://www.youtube.com/embed/EtGr1Uld1b4" frameborder="0" allowfullscreen></iframe></p>
 
 </div>
<?php
}

// Removes default welcome panel and instead adds in my_welcome_panel.
remove_action('welcome_panel','wp_welcome_panel');
add_action( 'welcome_panel', 'my_welcome_panel' );

Remove the balloon, the blue background image and close panel x icon. For that we need to use php function code.

In newer versions of WordPress the Welcome Panel has received a blue background and balloons. It is a bit tricky to remove. I located the code in this welcome balloons WordPress Trac ticket.

/* Remove the balloon, blue background and close panel icon in the Welcome Panel. https://core.trac.wordpress.org/ticket/55217#comment:2 */

add_action( 'welcome_panel', 'my_welcome_panel' );

function dashboard_remove_welcome_balloons() {
	?>
	<style>
		.welcome-panel::before { display: none; }
		.welcome-panel { background: none; }
		.welcome-panel-close { display: none; }
	</style>
	<?
}
add_action( 'admin_head', 'dashboard_remove_welcome_balloons' );

The result:

In this updated Dashboard widget tutorial I skipped adding these gists from the earlier tutorial.
https://gist.github.com/paaljoachim/f9f79b70d0e919bc6f17#file-welcome-panel-php
Original tutorial was written 20 June 2016.

Resources:

https://www.cssigniter.com/make-wordpress-dashboard-widget/
https://www.codementor.io/wordpress/tutorial/detailed-guide-adding-custom-widgets-wordpress-dashboard
https://code.tutsplus.com/articles/customizing-the-wordpress-admin-the-dashboard–wp-33110
https://markwilkinson.dev/2015/using-the-wordpress-admin-for-lead-generation/
https://www.engagewp.com/how-to-create-full-width-dashboard-widget-wordpress/