If you have a little HDD storage case (very cheap to buy) you can connect you new HDD to your current rig then run Disk Utility format and then go to 'restore'. There you can drag your old HDD icon to the "FROM" box and the icon for your new HDD to the "TO" box and hit enter. About 4 hours later if you are using a USB cable you will have a copy of your old HDD. All you need do them is follow the instructions to switch HDD's. If your old one is still working Ok you can put it in the storage case and use it for backup.
Answer to: How do I install the os after I replace my HDD?