Do I install Mac OS on SSD before or after I install it into my iMac?

The age of the system has a bearing on how I proceed.

In your case I recommend you use the case to hold your new drive and then run from your current drive Disk Utility to format it GUID and then run your OS installer to install your MacOS. Once the drive is finished your system should be booting up under it, if not test it by restarting your system and pressing the Option (⌥) key to gain access to the Startup manager. Finish updating your OS going to the Apps store getting all of the needed updates. Don’t forget to install a good antivirus and malware app with the latest profiles.

Take advantage of the Migration Assistant application to move your user accounts, apps and user data over.

References:

• Mac startup key combinations
• How to move your content to a new Mac - A good quick guide
• How to transfer your old Mac's data to your new Mac - A deep dive into how.

Now the fun begins! Make sure you follow this guide on how to safely remove your display to gain access to the drive bay iMac Intel 21.5インチ EMC 2544 のハードドライブの交換. If you hadn’t gotten the full kit from iFixit you really need the adhesive strips and the special tool to open the system don’t cheat! Its very easy to damage the display if you use anything else! [リンクされた製品が存在しない、もしくは無効: IF173-005]