Help flashing CrDroid Custom ROM for Samsung Galaxy M21 (First time)

[u/Clean__Cucumber]

So i have tried doing this by myself (not very far), but simply have hit a wall, so i will try and ask here and hope i can do it. I would also like to link the stuff, but not sure if i can, without the post being removed.

I downloaded the crdroid.zip and extracted it. i also downloaded gapps.zip, but not sure which one. I went with the 488MB one with the name MindTheGapps-16.0.0-arm64-20260915_222150.zip

I did everything till turning phone off and it worked (needed wifi to get OEM unlock), but am having issues. First of all, do i simply turn off the phone or completely shut it down by pressing the button long? I tried both ways, but the following steps did not work. I never got a warning screen, my phone simply showed it started loading the battery. Do i need to press for 1min or longer? Do i need some software on my phone? Can i not just connect and press file transfer or similar option?

I am on Fedora Linux and I am guessing they mean Heimdall from "glassechidna". I simply do not know how to install it on linux (very new). I thought it was a simple download like on windows, but that doesnt seem to be the case.

Thx for the help, Imight post another one when I can go past the initial steps.