Noted That:

To install correctly this wp-steam-auth.zip.
Fisrt Download the wp-steam-auth.zip to your computer
Extract/Open wp-steam-auth.zip to Your Computer.
Then, Find readme.txt file inside wp-steam-auth.zip and Open readme.txt.
Now, Read the Requirements of this plugin. Which Wordpress Version and PHP Version are required to run this Plugin in Your Wordpress Site.
Then, Follow the the Tips Below.

Start the Tips:

1. First Download "wp-steam-auth.zip" Plugin to your Local Computer. (Click Download)

2. Then, Login to your "yourdomain.com/wp-admin" Dashboard.

3. Then, Click on "Plugins" + "Add New" from left sidemenu of Dashboard.

4. Now, Click on "Upload Plugin" button.

5. Now, Browse "wp-steam-auth.zip" Downloaded plugin from your computer, Where you downloaded wp-steam-auth.zip According to Step – 1 Above then, click on "Install Now"

6. Now, Click on "Active Plugin"

7. Then, See left sidemenu. "WP Steam Auth" folder is added on left sidemenu. Now, Click on "WP Steam Auth" folder.

Noted that: If you do not see "WP Steam Auth" folder on left sidemenu then, see at left sidemenu "Settings" or "Tools".

8. Now you configure yourself oR Watch video tutorial below about WP Steam Auth Configurtions and Settings or How to work "WP Steam Auth" in your WordPress site.

After Activated Plugin According to Step-6 then,

Go to "Plugins" + "Installed Plugin" from Wordpress Admin Panel Leftside Menu.
or Direct go to: https://yourdomain/wp-admin/plugins.php
Then, Find "WP Steam Auth" Activated Plugin from Plugin List.
Then, Click on "Settings" from Plugin that is WP Steam Auth
Now, Edit/Add/Config the setting and Click on "Save Changes" button,

Guide

Guide
WordPress Install Upload the plugin files to the `/wp-content/plugins/wp-steam-auth` directory, or install the plugin through the WordPress plugins screen directly. Activate the plugin through the ‘Plugins’ screen in WordPress Setup your Steam API via: http://steamcommunity.com/dev/apikey Use the Settings->Steam Auth screen to configure the plugin Add the shortcode [wp_steam_auth] in your page or use available php functions Available Shortcode `[wp_steam_auth]` `[wp_steam_auth login_text="Login via Steam"]` `[wp_steam_auth login_class="my_class1 my_class2"]` `[wp_steam_auth login_image="http://..."]` `[wp_steam_auth logout_text="Logout"]` `[wp_steam_auth logout_class="my_class1 my_class2"]` `[wp_steam_auth logout_image="http://..."]` `[wp_steam_auth show_sync="1"]` `[wp_steam_auth sync_text="Synchronize"]` `[wp_steam_auth sync_class="my_class1 my_class2"]` `[wp_steam_auth sync_image="http://..."]` Available PHP Functions

WordPress Install

Upload the plugin files to the /wp-content/plugins/wp-steam-auth directory, or install the plugin through the WordPress plugins screen directly.
Activate the plugin through the ‘Plugins’ screen in WordPress
Setup your Steam API via: http://steamcommunity.com/dev/apikey
Use the Settings->Steam Auth screen to configure the plugin
Add the shortcode [wp_steam_auth] in your page or use available php functions

Available Shortcode

[wp_steam_auth]
[wp_steam_auth login_text="Login via Steam"]
[wp_steam_auth login_class="my_class1 my_class2"]
[wp_steam_auth login_image="http://..."]
[wp_steam_auth logout_text="Logout"]
[wp_steam_auth logout_class="my_class1 my_class2"]
[wp_steam_auth logout_image="http://..."]
[wp_steam_auth show_sync="1"]
[wp_steam_auth sync_text="Synchronize"]
[wp_steam_auth sync_class="my_class1 my_class2"]
[wp_steam_auth sync_image="http://..."]

Available PHP Functions

steam, authentification, users, synchronize, login, register,

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Reffered: https://wordpress.org/