The short answer is: you don’t, it’s already a master!
All IPA servers are masters, and equals. Some are just more equal than others. The distinguishing factors are: which was the first master installed and does this master have a CA?
In any IPA installation you absolutely want > 1 masters running a CA so you don’t have a single point of failure. When installing a new master this is not done automatically. You need to add the
--setup-ca flag, or run ipa-ca-install post-install.
The first IPA master installed is distinguished by two tasks it is responsible for: generating the CRL and renewing the CA subsystem certificates. See the IPA wiki for details on how to switch the master responsible for these at https://www.freeipa.org/page/Howto/Promote_CA_to_Renewal_and_CRL_Master