How to restore os drive from external drive?

I have a 2011 mac air and wanted to upgrade from version 10.6.8 to el capitan but with installation it keep saying os x could not be installed ..... file system verify or repair failed. My actions to correct this was to set the date (it was ok), and to do a disk repair in disk utility terminal which also failed ......."first aid process has failed"

So I was in this loop of instalation that keep failing and repair that keep failing.

I then found a very helpful solution on the apple community forum that helped me doing a backup to a external drive (first formated to HFS) skipping "bad sectors" this is the command used:

dd bs=4096 if=/dev/disk0 of=/Volumes/dev/disk3c2/Temp.dmg conv=noerror, sync Although the backup took very long it was eventually done successfully.

The next step was to restore the os drive from the backup external drive and the following command was sugested that I've tried many times but it keep failing:

"Restore > from imagedisk3s2" where source = this .dmg and target = The bad OS drive

It keep saying: Restore > from imagedisk3s2 command not found

I've then play around for many hours inserting space, upper cap's, various names like Backup disk etc. after the word image but without any success.

I will very much appreciate good sound advice ...... please if you can give it step by step for an old pensioner on the wrong side of sixty to understand and to do accordingly

I see forward to have my mac on el capitan and to have fun again