1. First, you need Boot-Repair. You can get it here.
2. Burn the ISO image into a USB drive. I used Universal USB Installer.
3. Using Notepad, edit the file "/boot/grub/grub.cfg".
Replace "gfxmode=auto" with "gfxmode=1920x1080".
Replace "quiet nosplash" with "nomodeset=1 xforcevesa".
(Actually found this tip here.)
4. Restart the computer into BIOS with the USB drive still plugged in. The easy way to do this is to shut down the computer. Then start up the computer, press F2 once the "Lenovo" screen shows.
5. Under "Boot", choose "Legacy" (instead of UEFI). For priority, choose "Legacy" too. Save changes and exit (press F10), which will restart the computer. Press F2 again to enter BIOS because you still need to change the boot priority.
6. Under "Boot", enable USB booting, and in the priority for legacy devices, move USB to the top (using F6). Once again, save changes and exit (F10).
7. The system should now boot into Boot-Repair. Click on "Recommended Repair" and let it run. Along the way, you may need to open up a terminal and run some commands. Do so. Once Boot-Repair has done its work, click on the power button at the bottom right of the screen. This shuts down the computer. Remove the USB drive.
8. Start the computer, press F2 to enter BIOS again.
9. Under "Boot", change priority to EFI.
10. Under EFI devices, there should be two of them. Go to the second one, press F6 to bring it up the list. Save changes and exit (F10).
11. You should now boot up into GRUB.
Hope it helps! Point 3 above should work for laptops using newer Nvidia cards, though you should change the resolution to whatever is your screen's actual resolution.