I used below instructions to flash LineageOS on a stock Samsung Galaxy S7 (herolte, arm64, exynos 8890). no rooting was required. If you intend to follow these instructions, do this at your own risk – this will surely delete all data on your phone and void manufacturer’s warranty. Also, there is always a risk of bricking your device. Proceed at your own risk!

1. Enable Developer Options. (Settings > About device > Software Info > tap Build number 7 times. 
2. In Developer Options enable USB Debugging and OEM Unlock.
3. Shut Down the device.
4. On your PC download ODIN 3. (I used version 3.12.3). Also download TWRP Custom Recovery image in .tar format (I used version 3.1.1-0). 
   ODIN is a ROM flashing tool for Samsung devices that we’ll use to install Custom Recovery (TWRP) image. I initially tried to use Heimdall Suite, but it didn’t work with my S7. ODIN worked just fine.
5. Boot your Galaxy S7 into Download Mode (Vol Down + Home + Power).
6. Start Odin3 and connect the phone via USB cable. If Odin detects your phone you’ll see something like this:
   
   If that doesn’t happen, you may need to install Samsung Drivers. In my case I went to Device Manager, found the phone under Universal Serial Bus controllers, right clicked on the device and selected Update Driver > Search Automatically. Windows 10 found and installed driver automatically. If you are using Windows 7, you may need to find the driver yourself online…
7. Click AP button in Odin and select your previously downloaded TWRP .tar image. Press Start and wait for the TWRP Custom Recovery to be installed.
8. Shut down the phone (Vol Down + Power) and then boot into Recovery Mode (Vol Up + Home + Power). This should load just installed TWRP Recovery.
   
9. Go do Wipe and perform 2 wipes – Factory Reset and Format Data.
   This will delete all data from your phone.
10. Now using Windows Explorer upload 2 .zip files to the phone – LineageOS image (I used version 14.1) and appropriate Google Apps image (in my case it was ARM64 / Android 7.1 / nano).
11. Using TWRP first install LineageOS image, then without rebooting the phone, Google Apps image.
12. Restart the phone and it should now boot in LineageOS.

Update: I had an issue with this LineageOS 14.1 installation where audio level was very low in recorded videos and recorded audio clips. This didn’t affect voice calls. Without finding a quick solution I replaced LineageOS with Android Nougat 7.0 ROM by ambasadii. It’s basically a Samsung ROM that was cleaned from knox, crapware, etc. and with bunch of added (optional) customisation. Google Apps are built-in so no need to download this separately. You can choose whether to enable root during installation. I didn’t have any issues with this ROM and it resolved abysmal battery life I had with stock ROM.