If your main domain is foo.x10.mx and you want to add a subdomain bar.foo.x10mx ,
log onto cPanel ,
right side, 2/3 way down
'Domains' section
'Subdomains' icon ... click
fill in 'bar'
standard is to have the content in public_html/bar ... ie subdirectory of the main site named for the subdomain, but you can change that if you want