how does one Lulu?

Yep, that's how it is for all supports.

Lulu can be seen SIMILAR to Lux, Malz, and Brand. Being that she has okay/pretty good ap scaling and as such can be turned around into a more damage-heavy support if your adc isn't making the cut.

However, even if you're adc is falling behind, I recommend trying to help them get back up. Such as helping them ward so if they're trying to farm near a tower they can potentially see if they may get dove, and if you're close by going to them to try and help. Just because they're behind doesn't mean they can't get back in the game, all they need is a little help.

And always buy support items. I've seen supports basically say "screw it" the first time their adc dies and then go full ap all because of one mess up. There's a few good ap items that work well with supports like Lulu, such as Athene's.

If the enemy team is very dive-heavy, go for a Locket. Even if another teammate has it, Locket is good to have, especially if that other teammate isn't there when a fight breaks out. Plus you then have a full team shield. I usually pair it with Spirit Visage or Knight's Vow, depending on if the enemy team is ap or ad heavy, for the extra buff on the shield.

When playing her, if there's a gank and the enemy dives, this is just me but I prefer to polymorph whoever is taking turret aggro. They become unable to flash, so you and your adc can damage them and they can't do anything or escape until the polymorph ends. Of course if there's someone who's more of a threat, you can polymorph them as well. Sure, speeding up your adc/teammate is great, but there's a lot of times a polymorph will be a lot better than a speed boost.

Her ult, however, can also be used to save people and not just disengage/engage(Yasuo for example). I've used it quite a few times to save someone from a Karthus/Lux/Draven ult because my shield wouldn't have been enough. Yes, you lose a knock up and a heal. But if whoever you used it on lives, that's one less death timer to wait out, meaning one more person available if a fight breaks out. Yes, I'm probably going to get shit for saying to use it this way. But I've found it works well using it that way and not just as a tool for team fights.