Before knowing the answer to the question how to install Wildcard SSL certificate for sub domains, have a look at the below given two scenarios.

Scenario 1:

When you have hosted all the domains in the same server, you just need to install Wildcard SSL certificate to that server alone.

Scenario 2:

When you have hosted the domains in the different servers, you need to install the Wildcard SSL certificate individually for all servers.

Refer the below tutorials to know the steps to install a SSL certificate.