How to create subdomains using htaccess?

First, you should set all subdomains to one directory in your web server config.

like this one below: 

ServerName  mydomain.com

ServerAlias   *.mydomain.com

Second, add this code to your .htacess file

RewriteEngine on
# SUB DOMAINS
RewriteCond %{http_host} !www\.([a-z0-9-]+)\.([a-z0-9-]{2,5})$ [NC]
RewriteCond %{http_host} ^(www.)?([a-z0-9-]+)\.([a-z0-9-]+)\.([a-z0-9-]{2,5})$ [NC]
RewriteRule (.*) /clients/%2/$1 [L]
# /SUB DOMAINS 

Finally, you should create a /clients/ folder in the main directory. Now every folder under /clients/ folder is a subdomain. 

For ex. If you have a test/ folder under the /clients/ folder, you can call the folder test.mydomain.com