Have a USB drive or SSD with same or larger capacity than your SD card.
When using an SSD SATA to USB adapter make sure it is fully UASP to get max. transfer/access speed. Here are some hints  - also this site has more details as I do provide below.
It goes like this:
Make a full backup of your SD-card (eg. with Win32DiskImager (<- easiest), Clonezilla, ...) to your desktop PC.
Put that backup at your USB drive / SSD (again using Win32Diskimager or the tool of your choice) on your desktop PC.
Resize the USB partition (only if the USB drive is larger than SD card).
Advise the pi to use a different partition after initial booting from /boot, handing over to the USB drive.
When you are done with the backup (Step 1 and 2 from above) attach both (SD card and USB drive) to the Rpi.
Remove any additional USB storage attached to the pi to avoid confusion.
Nothing has changed up to this point.
Note: Following steps are the crucial part - but no worries you have a backup. If something derails, you can back out by using the backup.
Step 3: Optional: Resize the USB partition (if USB drive is larger than SD card).
@beldar I don't notice tearing with SNES, NES, Genesis, Neo Geo, or Playstation. But I do notice it on Mortal Kombat 2 arcade which does run at a funky refresh rate. I also noticed it very badly with lr-flycast. Soul Calibur, MKGold, and NFL Blitz all had really bad tearing.