#lmtt

first, know why YOU love your ships! for me, i've been passionate about luisley for about 7 years now, so i've had a lot of time to figure out why it's so special to me (also why it comes so easily in writing them). this kind of leads into a few of the other tips i'll mention, but it's important to know WHY you, personally, love the ships you ship. what is it about luisley and mareach that you love? whatever the answer is, use that to help guide your writing. for example, i love how peasley and luigi are opposites in many areas of their personalities, but also how those opposites can bring out the best in them when they're together. :]

don't forget the little details. subtlety is key! pick out some things for each of the characters you're writing about that only a significant other might notice, like in LMTT where i have peasley notice luigi's quirks (like his mustache when he lies). i find there's something so romantical about a partner (or partner to be) noticing the little things that others might overlook. you can make these up, go off of canon, etc. etc.! have fun with it ^-^

read, read, read! this appears in a lot of writing advice lists so i won't stick here too long and bore you, but reading others' ship/romance works really helps to get the inspiration train a-rolling, and it will also help you realize what you do and don't like!

don't be afraid to let them disagree a bit. for this, i'm referring to your additional adjective in your original question: healthy. something really important for fictional (and real) relationships is the ability to communicate and leave room for disagreements. healthy relationships don't need to be lovey-dovey 24/7; especially in longer fanfic, there are bound to be moments where the members of your ship will not agree (though this has not stopped me from writing pure tooth-rotting, lovey-dovey fluff one-shots at times in my life, haha). the way one represents how ships handle conflict opens the door to a lot of interesting and compelling dynamics, methinks! this pops up in LMTT as well a few times, like in chapter 2. (side note: this does not mean you should feel pressured to have your couple arguing every page, i only mean that you shouldn't shy away from a moment where, organically, you think the members of your ship might disagree and/or want to go down different paths)

experiment! see which tropes you like! play around with your characters! if you placed them in a white room together, could you write a natural conversation with them? what circumstances could you add to that white room that would increase their chemistry? what setting does your ship thrive in? what setting do they NOT thrive in? doing little one-shots and experimental paragraphs to test out what one does and does not like has always been a great way to not only see how i want to write something, but also to tell if it will be able to sustain a longer piece.

above all, have fun with it! i've found that if i try and avoid writing super calculated and stiff romance, it comes off as much more real and organic. this will take time and practice, but just remember to experiment, keep reading, find what you like, and continue to absorb why it is that you love the ships you ship. :]