You would need to run some sort of backup client (or use Windows' own) on the computers, in which you create a backup set defining the files to back up and where to store the backup data.
You don't mention which OS the computers are running, but I am assuming Windows. Synology has an article explaining how to set this up using Windows 7's backup feature, which is also available on later versions of Windows.
The steps are outlined below:
- Run Backup and Restore (Windows 7) on the computers and set up a backup job
- Choose to save the backup data on the network
- Enter a UNC path for a share on the NAS, eg. \\NAS\Backup
- Enter credentials that has read and write permissions to the specified share
Steps 2-4 would be more or less the same if you choose to use a third party backup client.
Having said that, depending on the number of computers you need to back up, it might be beneficial to deploy a backup server which makes managing and monitoring backups much easier. You could still configure the NAS as storage (if supported by the backup client) as well as storing the backup data on the server for redundancy.