macos – Can not install Mac OS X Lion due to disk errors

I'm trying to install OS Lion after I accidentally erased my drive in order to restart my iMac, which I did without backing it up. On a separate USB drive, I have formatted and installed a Lion .dmg file in order to reinstall the software on my computer, which is currently launching the recovery mode due to my missing operating system. In order to download my operating system from my USB drive to my hard drive, I think I have to partition and erase my drive, but the error message stating that the drive can not be dismounted is probably due the fact that the volume is used. I've been trying to force the drive to dismount a few times and I'm stuck on this issue for days. Please help. I just want to use my computer normally.