To learn the way, first you must find out the ultimate point

Ah, starting with the end-point. As they say in some traditions, 'the goal is the path.'

hearing sound and seeing form

are inconceivable.

It's a critique of dualism: from the first-person perspective, there is no 'hearer' of the sound that is found, just 'sound.' Your ear and that noise are held in the same 'space' and aren't two separate things, appearance-wise.

If you discuss high and low

based on words,

it’s just like before you were enlightened.

If your discussion's foundation isn't experiential but is rather verbal/textual, then to somebody who 'gets it' your discussion sounds 'just like' someone who didn't get it experientially but was just quoting.

There is a road to emptiness;

everyone arrives.

The realization of emptiness, from the point of view of the individual, envelops everyone and everything. So reality has 'arrived,' everyone has arrived to you.

Those who arrive then realize the excellence of the aim.

Pretty self-explanatory.

The mind ground does not grow

useless plants and trees

Perhaps this is a reference to the notion of the 'mind ground.' Some people can take metaphors too literally, and fall into foolishness by thinking that the mind is literally like soil that grows trees and plants. Other texts in Tibetan traditions mention students who took metaphors too literally, when they said the mind is LIKE a sky. The sky isn't aware, but some students take a metaphor and go all the way with it.

naturally the body spontaneously

radiates clear light.

That's just a description of what it feels like sometimes during dharma practice.