Background
In a previous article I showed how to detect if Instant File Initialization (IFI) has been set.
If IFI isn’t set and is required then the following steps are needed (this example uses Server 2012 R2).
Locate and execute the Run command:

Locating the ‘RUN’ command
and within it, run ‘secpol.msc’, which launches the ‘Local Security Policy’ control:

Local Security Policy control
Under ‘Local Policies’/’User Rights Assignment’, locate ‘Perform volume maintenance tasks’:

Local Security Policy Summary
Right-click on this and select ‘Properties’:

Right-click on ‘Perform volume maintenance tasks’
In this screen-shot I have already added the service account details, via the ‘Add User or Group’ button.
I found the required service account details by running this query:
SELECT d.servicename , d.service_account , d.instant_file_initialization_enabled FROM sys.dm_server_services AS d;Once the change has been applied the server will require restarting to actually implement the change.