How can I make restore points in external hard drive from windows 7

You have to be clear about what you mean by a "Restore Point".

If by a restore point you are referring to the small key files backup made prior to a system change this has to be stored in the hidden System Volume Information folder on the hard disk the file is actually held on, you cannot change this storage point (normally the C: drive). The only option you have is to define the amount of space to be reserved for this purpose. Once you have set that then restore points are held on a FIFO basis where when the space is too full for a new restore point the oldest is dropped off.

The alternative is going to be a system image. This is obviously going to be very large for each backup so if you are going to be doing a number then you consider doing incremental backups. That way you do one full backup and then regular or event based backups of only the files that have changed. These backups are also defined as available points in the System Restore utility.