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There is not such a thing as delegating an individual record, a delegation always hands off control for a portion of namespace to another set of servers. That set of servers has control over that subdomain and anything below it, unless it delegates a portion to yet another set of servers.
The link you referenced does talk about the two general approaches for doing GSLB with external GSLB services (as opposed to integrated ones like Infoblox DNS Traffic Control (DTC)). Historically, you could do a delegation for each GSLB object pointing to the GSLB servers or you could create a dedicated subdomain for all GSLB records and then use CNAMEs to control what records are directed to the GSLB namespace. In my experience, the subdomain approach is a much better long-term strategy. Once the subdomain is delegated, everything else is just record management, which means you don't need to do service restarts every time you need to add a GSLB record, which you would if you are using individual delegations. I would highly recommend considering the subdomain approach, or to use DTC, which allows you to do GSLB directly within the domains and doesn't require any additional systems.
If you do want to continue on the path of doing individual delegations; however, you simply do a delegation for that name pointing to the GSLB servers. Navigate into the domain that the delegation would fall under, switch to the "Subzones" tab, and then use the arrow next to the + to add and select "Delegation". See the screenshot.
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