I highly recommend grabbing a copy of httpd and Relayd Mastery, even config aren't valid now, it provides simple yet effect overview of topics (for instance TLS, no more SSL)
OpenBSD comes with acme-client copying letsencrpyt example config
/etc/examples/acme-client.conf will do and
acme-client -vv domain.tld
should generate fine.
Two things to look for if it fails,
Add certificate path (not present in example as of now)
domain certificate "/etc/ssl/displ.nl.crt"
For subdomains, after setting A record in domain registar Make sure it works with random caps.
Blogs to look for,
My hearty thanks to letsencrypt for providing great service, making TLS an industry standard.