How does Ukraine start negotiating with a country that has invaded theirs?
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.
You think Ukraine should make concessions with the country that invaded them?

For the sake of peace, yes, I think they should be willing to make concessions. That’s how negotiations work.

If you refuse to offer anything you aren’t really negotiating. You’re just issuing demands with no exchange.

If someone came into your country and started to rape, kill and kidnap your people would you roll over and give them whatever they wanted to stop doing that?

If the alternative was that they would rape, kill, and kidnap my people for the next 20 years without end?

I’m not willing to fight this war to the last Ukrainian.

Say they do negotiate peace, what do they do next time Russia wants to invade?

Give more up?

It is possible to include certain guarantees within a treaty to make it painful for either side to break it, or to make breaking it extremely difficult. That’s what Ukraine would have to demand from Russia - some kind of leverage or collateral to guarantee the peace holds.
Ukraine would have to trust Russia would comply and they haven’t historically.
No, trust is for fools. They need some kind of mutually assured destruction so that neither side can ever betray the peace treaty.