Activating standby DR after opening it in read-write mode
If you had flashback enabled or created a restore point before activating the standby database, you can flashback the database to that point/SCN, and the standby database can continue from there like nothing happened.

You can do this manually, or if you have Data Guard Broker configured, you can use the REINSTATE DATABASE command to do this.

In case you have not prepared for this, you will need to rebuild (duplicate/restore) the standby database.

Next time do not do this, there is a feature called Snapshot Standby designed for testing tasks like this, that works just as I described above.