I asked my husband this question and his answer was “it just happens”, so that’s helpful
I think for us, it starts with making one another feel appreciated and supported. We both WFH, so one makes breakfast tea/coffee and the other makes lunch. He cooks dinner, I clean the home. We talk to one another about our problems during the day as well.
When it comes to sex, sometimes it starts with a joke. For example, if my husband has had a report of a leak then I might say (in a really corny, totally suggestive voice) “I know somewhere that’s wet that the engineer can’t go”, and my husband, knowing exactly what I’m referring to, will say “really? Perhaps I can take a look”, knowing full well that’s exactly what I want to happen.
Sometimes it will start in more direct ways. I might ask him what he wants for lunch, to which he’ll say “you”, “you, naked” etc.
Then there have been the times when we’ve been triggered by something on TV. I remember watching the orgy episode of Open House: Great Sex Experiment and we kinda felt like we were being left out, so we had some fun of our own in the lounge. We also watched a film where the woman cheats on her husband and I admitted to my husband that the thought of having sex with a stranger turned me on. I now “cheat on my husband, with my husband” from time to time. Our marriage is fine, but the sex when I “cheat” is spectacular. He doesn’t treat me like his wife either, he treats me like cheap, easy meat and I love it, but afterwards he is so affectionate and sweet with me so I know it’s just rolepley
There is our Kinky Fuckery Thursdays as well. They are scheduled, once a fortnight on a Thursday. There’s no initiating there, that’s basically “you know it’s gonna happen” sex
Lastly, is there a time when it does just kinda happen? For us, Saturday mornings are that one time. There’s no plan or arrangement, one of us is awake, the other wakes up, we kiss and… well, you can fill in the rest
I hope some of this has inspired some ideas