[Q] Failed kernel flash, phone wont boot.

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I wanted to flash the Siyah kernel, but failed. Now the phone is always stuck at the Samsung logo, and I can't even enter Recovery mode, only Download mode which implies I must use Odin.

Any advice?

Flash a stock rom in Odin, factory reset. Clean slate. Then try flashing whatever rom/kernel you want again if you're still keen.

Edit - Or...Flash stock rom, root phone, restore Nandroid backup (assuming you have one).

Any guide for using Odin to do just this?

Here. But don't let me stop you searching for it :-|