This video will show you how to register personal nameservers for a domain.
Overview for the domain DNS options (including ‘Custom DNS’ option) at Namecheap: https://youtu.be/YbJr7YPmDsQ
If your personal nameservers are hosted with Namecheap, feel free to check these guides on how to set them up on your server:
How to set up private nameservers (Business packages) https://www.namecheap.com/support/knowledgebase/article.aspx/10099/32/how-to-set-up-private-nameservers-business-packages/
How to set up private nameservers (Reseller packages) https://www.namecheap.com/support/knowledgebase/article.aspx/9559/32/how-to-set-up-private-nameservers-reseller-packages/
How to set up private nameservers (VPS and Dedicated servers) https://www.namecheap.com/support/knowledgebase/article.aspx/207/48/how-to-set-up-private-nameservers-vps-and-dedicated-servers/
How do I register personal nameservers for my domain?