Step-by-step instruction

Step 1

At the root of the project (@app) add a .htaccess with the following content:

<IfModule mod_rewrite.c>
    Options +FollowSymlinks
    RewriteEngine On
</IfModule>

<IfModule mod_rewrite.c>    
    RewriteCond %{REQUEST_URI} ^/(admin)
    RewriteRule ^admin/assets/(.*)$ backend/web/assets/$1 [L]
    RewriteRule ^admin/css/(.*)$ backend/web/css/$1 [L]
    RewriteRule ^admin/js/(.*)$ backend/web/js/$1 [L]
    RewriteCond %{REQUEST_URI} !^/backend/web/(assets|js|css|js)/
    RewriteCond %{REQUEST_URI} ^/(admin)
    RewriteRule ^.*$ backend/web/index.php [L]
    RewriteCond %{REQUEST_URI} ^/(assets|css|js|images)
    RewriteRule ^assets/(.*)$ frontend/web/assets/$1 [L]
    RewriteRule ^css/(.*)$ frontend/web/css/$1 [L]
    RewriteRule ^js/(.*)$ frontend/web/js/$1 [L]
    RewriteRule ^images/(.*)$ frontend/web/images/$1 [L]
    RewriteRule ^(.*)$ frontend/web/$1 [L]
    RewriteCond %{REQUEST_URI} !^/(frontend|backend)/web/(assets|css|js)/
    RewriteCond %{REQUEST_URI} !index.php
    RewriteCond %{REQUEST_FILENAME} !-f [OR]
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^.*$ frontend/web/index.php
</IfModule>

Where admin - path to the @backend/web.

Step 2

In the folder @frontend/web add the file .htaccess with the following content:

RewriteEngine on

# if a directory or a file exists, use it directly
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# otherwise forward it to index.php
RewriteRule . index.php

Step 3

In the file @frontend/config/main.php in item components of array add:

'request' => [
    'baseUrl' => '',
],
'urlManager' => [
    'enablePrettyUrl' => true,
    'showScriptName' => false,
    'rules' => [
        '' => 'site/index',                                
        '<controller:\w+>/<action:\w+>/' => '<controller>/<action>',
    ],
],

Step 4

In the folder @backend/web add the file .htaccess with the following content:

RewriteEngine on

# if a directory or a file exists, use it directly
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d

# otherwise forward it to index.phpp
RewriteRule . index.php

Step 5

In the file @backend/config/main.php in item components of array add:

'request' => [
    'baseUrl' => '/admin',
],
'urlManager' => [
    'enablePrettyUrl' => true,
    'showScriptName' => false,
    'rules' => [
        '' => 'site/index',                                
        '<controller:\w+>/<action:\w+>/' => '<controller>/<action>',
    ],
],

Well done)