I remember that I found it particularly difficult to install and boot into the custom recovery.
I've attached a picture what the custom recovery looks like so you can distinguish it from the factory recovery; then follow the steps in the instruction.
Maybe you could take a picture of the factory recovery screen first and also post it here so people can distinguish the two different recovery modes better.
Once lineage OS is installed the boot sequence will look similar to the attached pictures. Good luck!
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