The advantage of cloning is that it copies all of your data from the original HDD to the new one in one step. In order to do that, you have to place the the new drive or the original internal drive in an enclosure to make the clone.
If you cannot do that, then you will have to format and install an OSX on the new Seagate SSHD that you have placed on your Macbook. Since you indicate that you have Snow Leopard, you will have to use that disk for the installation.
Place the SL disk in the optical drive and start the Macbook holding down the C key.
When you get to the point where the display shows a menu bar, select UTILITIES.
From the drop down menu, select DISK UTILITY.
Select DISK UTILITY>ERASE and format the new drive to Mac OS Extended (Journaled).
Then continue with the installation of the OSX.
Once the OSX has been installed, you still have to transfer your user data to the new internal drive. You still will need an enclosure to do this.
Yes, you can use an external drive (one purchased with a HDD already in an enclosure) but you have to copy the data from the Macbook to the external drive with the original HDD still in the Macbook. Then you can insert the new SSHD in the Macbook, install the OSX, and then transfer your user data from the external drive
The other option is to install your original HDD into the Macbook and then clone it to the external drive. Install the new SSHD in the Macbook and then clone from the external drive to the Macbook SSHD. This is a two stem process eliminating a new enclosure.
I hope this makes sense to you.
Ciao.