Put the old drive into the external enclosure. Make sure you can boot from it. Install the new drive. Boot up holding down the option key and select the old drive. Go to Applications > Utilities > Disk Utility. Select the new drive and format it and give it one partition. Now You can either download a system from the Apple App store and install it and then use Migration Utility or Use a cloning program such as SuperDuper (free), and clone the old drive onto the new.
Put the old drive into the external enclosure. Make sure you can boot from it. Install the new drive. Boot up holding down the option key and select the old drive. Go to Applications > Utilities > Disk Utility. Select the new drive and format it and give it one partition. Now You can either download a system from the Apple App store and install it and then use Migration Utility or Use a cloning program such as SuperDuper (free), and clone the old drive onto the new.