You can easily ruin a harem if you neglect to add backstory and personality and make them too much of a yes-lady.
But then again, even if the harem is top-notch, most authors just plainly make the heroines background characters after their time/development/conquering is over.
Developing a harem is difficult in itself because you have to take into consideration that said protagonist of has to be likable yet can't be an every man. You also have to take your time and really put slot of thought into your development and ask what kind of girls would actually be willing to accept a scenario.
I mean, if the girls aren't broken or ridiculously idiotic from the start, they wouldn't agree to share.
But well, I'll offer a tip -
The best way to conjure a harem is to create a scenario which starts it off. Every girl involved needs a reason to show interest in the guy. It could be either for his kindness, manners, formality and so on. It doesn't always need to be something like because of his looks, charms, talent or even physique. So you should create scenarios that lead up to said girls developing an interest in this one guy
Also, try to keep the number of girls to a stable limit. To be honest, a huge harem of girls who all like one guy is going way too overboard. At least four or possibly five girls is enough. The reason I say this is because most of the harems I've read have the mcs attracting a multitude of girls not just from earth, but from Mars as well. I feel that's too much, way way too much.
I'd suggest you to read harem novels, you'll learn.
I think one of the absolute best harem stories I've seen has to be from Highschool DxD. The mc is a total pervert obsessed with the female form, yes, but he also genuinely cares enough about the women he knows not to try and take advantage of them. And despite the pervy actions and desire for a harem, his ideal is a massive number of women that love and care for him, and are cared for by him in return.
Especially interesting in this story, to me at least, is the fact that some of the mc's past experiences stunt the development of his relationships with the girls. He doesn't want to make a move, and he disregards the girls' very obvious advances as a joke, as a result of this really deep-seated trauma, and it genuinely takes a long time for him to find the inner strength to move past it and realize that he's kind of surrounded by a substantial number of girls that he really cares about, and that care for him in return, which is kind of his dream.
Oh, and the female protagonists are all pretty good. the first few that make up the main cast really feel like their own people, while most of the later ones are likely tacked on to fill the niche stereotypes for all the fanboys, but are still entertaining, at least. (All of this is helped by the fact that I absolutely love modern fantasy and later on in the story, the action just gets absolutely ridiculous, but again, that's just me.)
If I were to give you any advice, I'd just tell you to write good characters. Give the potential harem members a reason to love your protagonist beyond "the story demands it!" Try not to make your mc unlikeable. If the audience doesn't like 'em, they'll constantly question why all these girls/guys flock to em. And try not to make the mc clueless about the harem. If they'd make a choice, have them make a choice. If not, have a good reason.
Don't really know if there's anything else I can say. Good Luck?
I have too much timmmmmeee
