Put the SSD into the case.
Connect it to the Mac.
Open Disk Utility. Initialize it (erase it) to Mac OS Extended with journaling enabled.
Give it a name that is meaningful to you.
You now have a Mac-formatted SSD that is "empty" and ready for the OS install.
My recommendation would be to install either El Cap or Low Sierra onto the drive while it's still in the external enclosure.
If you do it this way, and something goes wrong, YOU STILL HAVE A MACBOOK THAT WILL BOOT AND RUN. Kabish?
However, I still recommend that you ALSO create a bootable USB flashdrive with the El Cap (or Low Sierra) installer on it. Installs just go better when you do it that way.
Connect it to the Mac.
Open Disk Utility. Initialize it (erase it) to Mac OS Extended with journaling enabled.
Give it a name that is meaningful to you.
You now have a Mac-formatted SSD that is "empty" and ready for the OS install.
My recommendation would be to install either El Cap or Low Sierra onto the drive while it's still in the external enclosure.
If you do it this way, and something goes wrong, YOU STILL HAVE A MACBOOK THAT WILL BOOT AND RUN. Kabish?
However, I still recommend that you ALSO create a bootable USB flashdrive with the El Cap (or Low Sierra) installer on it. Installs just go better when you do it that way.