The same way anyone starts negotiating with anyone: open a dialogue and exchange demands, and then work to make concessions and compromises. They won’t do that, though, because then the infinite money spigot from the US will shut off because America doesn’t want this war to ever end.

I doubt Ukraine will demand anything less than Russia pulling out of previous held lands. Russia will doubtless demand all lands it currently occupies (and maybe even ones it claims but does not occupy). Russia's demand could be cast as peace, though really it involves giving a massive portion of Ukraine to Russia. And if you're thinking that might be temporary... well, just ask Finland, which lost 9% of its territory to a peace agreement in the Winter War.

They’d each have their own demands, and then negotiations would be finding a middle ground between “Ukraine gets everything it wants” and “Russia gets everything it wants”

Why should Ukraine have to make any conversations at all? Russia invaded and took land and lives, and you think Ukraine should just give up some of that, just cuz?

Because otherwise the war never ends.

Are you willing to fight to the last Ukrainian?