From your Splunk Cloud UI, launch the "Universal Forwarder" app and this will bring you to a page with instructions on how to configure forwarding to your Splunk Cloud instance. This includes a link to a credentials package that you will need to download and install onto each forwarder (this is essentially an app which sets up your outputs.conf to point to the Splunk Cloud indexer cluster and also has the required security certificates).
Please note also that applies also to Splunk Enterprise instances (e.g. Heavy Weight Forwarders) that you would like to use to forward data to Splunk Cloud (i.e. this is not only just for configuring Universal Forwarders).