people overthink this one. in the godaddy dns panel you add an a record, host is the subdomain part only (shop, not shop.example.com), value is the ip, ttl whatever it lets you choose. that is it, thats the subdomain, it exists the moment dns resolves.
the part that trips everyone up is that creating a subdomain godaddy side does nothing on the server. your web server still has no idea the name exists. if it is nginx you need a server block with that name in server_name and a reload, if it is apache a vhost, if it is a control panel you add it as a site there. i have watched three people decide dns was broken when nginx was quietly serving the default site.
use a cname instead of an a record if you are pointing at another hostname, that is the optional second one, and never point a cname at a bare domain.
tl;dr add the a record, then tell the web server the name exists.
the only real gotcha left is wildcards, because a wildcard answers for names you never created and then you end up with
Source: r/SiteManagement · by /u/petra_ops