This is pretty simple and straightforward. You just need to grant access permission for SQL to user.
As any other asp.net web application kentico is having web.config file - in this file you are expected to specify an account, database name and database server name. These are used for accessing the DB.
Provided you are using SQL user authentication:
<add name="CMSConnectionString" connectionString="Persist Security Info=False;database=databasename;server=servername;user id=username;password=password;Current Language=English;Connection Timeout=240;" />
Provided you are using windows integrated authentication
<add name="CMSConnectionString" connectionString="Persist Security Info=False;Integrated Security=SSPI;database=databasename;server=servername;Current Language=English;Connection Timeout=240;" />
In either way you need to have this user created in your SQL and this user is expected to be owner of the kentico DB.