I would like to use 'ns1.mydomain.com' branding on the NS records for all websites I manage/own. So instead of ns1.linode.com to ns5.linode on whois record, mydomain name would show up.
How would you do that?

If I add A record on my DNS editor for mydomain.com. But I believe one of linode IP is IPv6. What's the procedure to correctly point my ns1.-ns5. mydomain.com to replace ns1.linode?

Thank you.

You will need to visit your domain name registrar's website and register your nameservers with them. Sometimes registrars call this "adding glue records" or "registering nameservers" or something similar. You'll specify your custom nameserver names (ns1.yourdomain.com thru ns5.yourdomain.com) and their corresponding IP addresses. You may then need to also specify that these new nameservers are the authoritative nameservers for your domain.

In the Linode DNS manager, you should also create A records for these hostnames and possibly additional NS records for your domain (which specify your custom nameserver hostnames).

Do you think your clients will care (or even notice or even be able to spell dns)? A simple whois will show who actually owns those IP's.

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Either provide enough details for people to help, or sit back and listen to the crickets chirp.
Security thru obscurity is a myth - and really really annoying.

haha, yes, I know they don't care. And they dont even know how all this works.
But for my knowledge & experiments I'd need to implement it. (as a part of my learnings)