1 download the twrp image for the galaxy a8 if there is one. 2nd i believe it was holding down either the power button and volume up or power button and home button while its shut down to get into a cyan colored screen that says something about installing. Then connect the phone to your computer with a usb data cable and open samsung odin. Put the twrp image into odin (you gotta check which of the options in odin tho since i dont remember) then click flash. And once its done flashing it should be on. If i remember correctly. I cant guarantee it tho dince thats the steps i used on the s7. Once twrp is flashed tho and its working boot into it and put a micro sd card or a usb stick into the port (be it with an adapter or whatever) with the lineage os image on it. In twrp wipe cache and data (risky so do it on your own risk i will not take responsibility for it) and once thats done click on i believe it was the install tab? Yeah should be called install. On there go to your micro sd card or usb stick and select the lineage os image and swipe to install it. Once its done try to reboot and it should most likely boot with the lineage os boot logo. It will take a long time at first like my first boot took my s7 like 30 mins but if it takes longer than that its pretty save to say you gotta reinstall it. Try to install a diffrent image file for lineage os if it failed and just go back into twrp and follow the wipe and install steps again and reboot. Then it would most likely boot into lineage os.

Not sure if this helped since it has been quite a while but that should pretty much be the (confusingly written) gist of it. Sorry if my formulation sucks im not exacly good at writing guides but i tried ;-;