From Info about Samsung Galaxy S8 - dreamlte (SM-G950F) we find the instructions to “Install via command line (for advanced users)” → Install /e/ on a Samsung Galaxy S8 - “dreamlte”
Maybe it looks a bit intimidating to start, but please try to break it down in you mind into easy stages! 
This is done in this tutorial [HOWTO] Install /e/ on a Samsung smartphone with Windows easily
Please allow yourself time to let your mind get used to it!
If it feels like you lost the phone – you definitely did not, it is quite hard to brick a Samsung.
Your device is only missing the Normal booting part – system.
The two modes you need are working fine, Download mode and Recovery mode.
Please be aware that you need the phone charged >70% to proceed. It can be reassuring to halt the phone (from Recovery mode) and see it charging correctly.
Don’t hesitate to ask again if you get to a block with the instructions – much better to make sure you can follow step to step before committing further, I think.
Good luck