Create a .htaccess file, copy and paste the below code in the .htaccess file and save it.

<IfModule mod_rewrite.c>
    RewriteBase /
    RewriteEngine On

    # Check if mobile=1 is set and set cookie 'mobile' equal to 1
    RewriteCond %{QUERY_STRING} (^|&)mobile=1(&|$)
    RewriteRule ^ - [CO=mobile:1:%{HTTP_HOST}]

    # Check if mobile=0 is set and set cookie 'mobile' equal to 0
    RewriteCond %{QUERY_STRING} (^|&)mobile=0(&|$)
    RewriteRule ^ - [CO=mobile:0:%{HTTP_HOST}]

    # cookie can't be set and read in the same request so check
    RewriteCond %{QUERY_STRING} (^|&)mobile=0(&|$)
    RewriteRule ^ - [S=1]

    # Check if this looks like a mobile device
    RewriteCond %{HTTP:x-wap-profile} !^$ [OR]
    RewriteCond %{HTTP_USER_AGENT} "android|blackberry|ipad|iphone|ipod|iemobile|opera mobile|palmos|webos|googlebot-mobile" [NC,OR]
    RewriteCond %{HTTP:Profile}       !^$

    # Check if we're not already on the mobile site
    RewriteCond %{HTTP_HOST}          !^m\.
    # Check to make sure we haven't set the cookie before
    RewriteCond %{HTTP:Cookie}        !\mobile=0(;|$)
    # Now redirect to the mobile site
    RewriteRule ^ http://m.example.com%{REQUEST_URI} [R,L]
</IfModule>

Replace URL http://m.example.com with your own URL