Generate SSL keys by running:
openssl req -x509 -batch -nodes -newkey rsa:2048 -keyout logstash-forwarder.key -out logstash-forwarder.crt
Copy the resultant .key and .crt files to roles/logstash/files/certs.
Copy vars/global_vars.yml.sample and customise it to suit your setup - it should be fairly self explanatory.
Add the logstash groups to your ansible hosts file
ansible-playbook logstash.yml