Use the SSL keys you generated for your logstash configuration (see here for instructions on how to do that), and copy the .key and .crt files you generated into roles/logstash-forwarder/files/certs.
Copy vars/global_vars.yml.sample and customise it to suit your setup - it should be fairly self explanatory.
This ansible config installs a .deb generated from the logstash-forwarder sources. You can generate your own by following the instructions here.
ansible-playbook logstash-forwarder.yml