First run this command in console:
certbot certonly --webroot -w /path/to/webroot/ -d domain.name
Then, create /etc/nginx/snippets/ssl-domain.name.conf
ssl on;
ssl_certificate /etc/letsencrypt/live/domain.name/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/domain.name/privkey.pem;