1) Make sure your back files are in a safe place
2) reinstall Oracle the same way you installed it the first time. (with the same SID and instance name)
3) make sure Oracle starts up OK with a 'blank' database
4) shutdown the database
5) copy your backup files (datafiles, control files, init.ora, redo log files, etc) to their original locations
6) If this is a cold backup, then you should be able to just
start the database. It is a hot backup, then you will need to perform recovery first.