It's the exact same steps as the guide for FreeBSD [0] but you do it with a NetBSD install image instead.

I maintained an installer [1] for these systems a while ago but the images may not be available anymore, not sure.

[0] https://www.linode.com/docs/tools-reference/custom-kernels-d...

[1] https://github.com/eatonphil/linode_deploy_experimental