Just the bare essentials below (with my own modifications):

openssl ecparam -genkey -name secp384r1 | openssl ec -out ec.key
openssl req -new -sha256 -key ec.key -nodes -out ec.csr -outform pem

Depending on certbots ACME challenge methods preferred:

sudo certbot certonly \
	--dns-cloudflare --dns-cloudflare-credentials cloudflare_id.ini \
	-d h.auyong.me --csr ./ec.csr
sudo certbot certonly -w /var/www/ayhx/_site/ -d h.auyong.me --csr ./ec.csr

Source: Using ECDSA certificates with Let’s Encrypt, 2016