Booting from an external hard disk
Here's what I used to do back in the day when hard drives were swappable in MacBooks (and PowerBooks before, I'm old.)


Put the SDD in an external exclosure and connect it to the Mac.
Clone the HDD to the SSD.


Use SuperDuper or Carbon Copy Cloner, both are great and have generous trial versions.
You now have a full backup AND a functional bootable drive.

Swap the HDD with the SSD. 


The HDD is in the external enclosure. You can use it as a backup, both SD and CCC let you update/sync your clone drive.
Your Mac is now on SSD. Everything works just like before only much faster.



That's it! Enjoy!