There’s 3 options for Leia:

Die offscreen between movies.
Recast/CGI.
Find a way to relocate her character out of the main events.

1 is the most likely. 2 would cause massive fan backlash. 3 would be incredibly awkward to write and would break audience investment moreso than TLJ did at times. JJ could do that, though, he has a habit of just not explaining things and just moving past quickly. Star Trek was full of that.

So it depends on whether there’s a timeskip.

If no timeskip, then probably something like it begins with Leia’s funeral, which wouldn’t require much cgi or actor standins and would probably be respectful enough for the fandom. Something that parallels Padme’s funeral, maybe.

But If there’s a timeskip of like 5 years or something, they can dodge that altogether. I just whipped this up as an example:

[Resistance ship: Admiral POE DAMAERON is making a speech on a slightly raised dais in front of the crowd; he’s still the brash young pilot we know but now has years of maturity and leadership under his belt. He stands tall and gestures with confidence.]

Poe: …it’s not over yet. We’ve got to make this one last push, we have to honor the sacrifices of everyone who came before us. Han Solo proved there is always another way out. Luke Skywalker showed that this new “Supreme Leader” is a joke. And General Organa…Leia…she showed us there is always hope; there is ALWAYS hope. The fact that we are here, now, proves they will live forever, and as long as we remember them, the resistance will never die! Today we strike back, today is when we show the galaxy that there are still heroes, and they fight along side us! Today we BECOME the heroes!

[Crowd cheering, raising weapons, clapping, FINN and ROSE especially. As everyone goes to their stations, FINN climbs up to the dais to talk.]

Finn: Great speech, Admiral.

Poe: [smiles] You don’t have to call me that.

Finn: Names are important, but so are accomplishments. [Looks over the bridge from their elevated angle] They all believe in you.

Poe: …I believe in everyone. We’re going to win this.

Finn: Yeah…we are. [Smiles with assurance from Poe’s leadership.] See you out there, Admiral. [Shakes hand and then heads off the bridge.]

[POE pauses, then heads into his cabin off to the side.]

[POE’s cabin: very sparse. He is sitting and talking to someone off-camera. Body language is completely different; he is slumped over, unsure, hunched over.]

Poe: Are we really going to win? Is this how you would have done it? What would you say to me right now? [Pause] Probably something about how stupid and reckless this plan was, but that I would do it anyways. [Chuckles sadly.] Sometimes I worry if victory is really worth the price. What hope is there when there’s nothing to hope FOR? Is it worth winning if you have to lose everything?

[Camera pan: He’s talking to a small white shrine, a few rocks, a small plant, a holographic picture of Leia, similar to those in Padme’s house, is there, smiling. It is the total opposite of KYLO’s shrine for VADER’s helmet, whereas that was black ashes, this has white salt from CRAIT.]

Poe: But I guess you kept going, even when you lost everything, didn’t you. That’s really what you taught me, why you entrusted this to me. It’s only been a few years but I always wondered why, why me. Did you question yourself like this? [Pause.] You did…that’s IS why, isn’t it; that’s why you trusted me, because I was willing to question myself but still keep fighting and finding a way out of things. Even when there was no hope back on Crait, you let my hope guide everyone. I regret not having the chance to thank you for that; losing you so soon after still hurts to this very day. [Tears up slightly, but regains composure and stands up in front of her.] But now I can thank you properly. Today will be our victory, to show that hope lives forever.

[He salutes her small grave.]

This is for you, Admiral Leia Organa.