Create DB Link

I'm not a big fan of creating a db link in the tnsnames.ora file because during clones you need to start messing around with custom files. My preferred method is using a bit of SQL to create the db link.

Using the details from the tnsname.ora file of the target database:
TEST=
 (DESCRIPTION=
  (ADDRESS=(PROTOCOL=tcp)(HOST=oradbtest.company.com)(PORT=1521))
 (CONNECT_DATA=
  (SERVICE_NAME=TEST)
  (INSTANCE_NAME=TEST)
  )
 )
I can now create the database link as follows:
create public database link TESTDBLINK connect to apps identified by apps using '(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=oradbtest.company.com)(PORT=1521))(CONNECT_DATA=(SERVICE_NAME=TEST)))';
Just remember you might want to not make it public and you may need to change user names and passwords to the target instance.