Pretty easy..
Download an FTP client i use flashfxp:
Flashfxp.com
Download, Install.
Launch flash fxp and press F8 for new connection.
Login to ovh manager then go to backup..
Get your backup space hostname + username.
Enter in all the relevant details in fxp such as hostname, username and password so it should look like this:
Click connect
And your done! you can drag and drop your files.. If you want to re download your files just right click the file you want and press "Transfer".
Please also note ftp backup will only work within your server. You will not be able to access it on your home computer.