Plot.

It sounds insultingly simple, but you have to have a longer running plot that keeps the characters together, interesting enough to keep the readers attention, and allows you to slowly change the descriptors used for the other part of the pair in anatural manner until they are in love enough to wax poetic over each other - if they are the type.

It's the reason I like some kinds of Romantasy over contemporary romance. The romance burns slower and more naturally over two inches of spine than a half inch.

Alright. I've spent way longer thinking about how slow burns work for longer then any reasonable person would.

The slow burn is all about the tension and the gap between the core three elements and the two characters.

Physical Emotional  Willingness

The most common usually would have a combination of emotional with some physical embodiment. So they start to like eachother maybe notice they're thinking about the other person. When the other person stands close their heart beats faster, maybe a casual touch feels electric. They want to be near the other person, feelings build, but you don't let them have what they want. You let them be close, find excuses for proximity but it's all about the almost, the nearly. Give them plot or character reasons why they can't be together or why they resist. That's where the willingness gap comes in.

You might think the burn completes with the kiss or finally being physical. It that's not what really completes the burn. 

I've read some fantastic slow burns that start with the physical. The couple deny their growing feelings instead and the willingness to admit is what creates the tension.

Declaration and pursuit of the relationship is what completes the burn. The choice. 

Both can be in love and want eachother and when you have that plus friction then you have a supernova. 

Because I'm a lot I created a slow burn tension calculator.  Don't ask me why I did this.

http://www.goodsecretsandbetterlies.co.uk

First of all, do you know why they were no lovers to begin with? Was there someone else in the picture? Why were they not attracted to each other to begin with? What happened that they became distant? What changed that they fell in love afterwards?

Here are some problems I see with your intended progression: - Friends don't have a spark, otherwise they only pretend to be friends but are aware that there might be more. If you want to go with that one, be aware that most of the time, one party is already interested and slow-burn ist much more difficult. - distance doesn't lend itself that well to slow-burn, because slowburn lives from small interactions; glances, accidental touch etc. Distance doesn't support the yearning very much, at least If there is emotional distance as well as physical distance. - Love: why not focus in that part exclusively? Can't you use the friendship and the distance as background for the characters? Think: they were friends, then one of them moved/declared their feelings, and now, months later, they meet again (inciting incident). Woah! But is the other one (still) interested? That's cliché, but not without reason.

My advice: Make sure the progression makes sense for a slow burn. Make sure the POVs make sense for a slowburn as well. Usually it's dual POV. Make the reasons why they were not immediately on track to become a couple believable and take your time crafting those characters. What are their flaws, what attracts them to each other (and what doesn't (in the beginning)?)

Good luck!

Read published slow burn romance! Especially if you haven't read and reread much before. It doesn't have to just be contemporary. I think Jane Austen counts as slow burn, for instance. There are books out there on how to write romance novels, your library likely has some.

I'd start with them being friends, forged that friendship, make it as part for their life or routine. Then they go to separate ways, starting their own life or anything, this can be your distant in the distance you look for, yet while they're struggle at their new life. They kept thinking about their friendship or remember, thinking about what would they do or what would the other person do, you know, missing the presence of the other but not yet love. This can be the slow burn part, at the same time the chance for their character arcs, let them each have their struggle and how the other cheer and support as friends, then an inciting incident happen when they are already where they want to be in life (got what they wanted in life or planned), and realized who was there all along and realized they want to have each other, which they actually already have, they went through the ups and downs of their reality together but apart, and now the stake should be higher, they want to be physically together, in each other vicinity, distance dont work anymore, not after the slow burn relationship they build after 2-3 years.

Idk if I have the right to call it slow burn. But... In my own experience what makes slow burn happens is... Everything else. So I would put romance as sub plot instead the main plot.

I've reached 400k words in a manuscript to finally make the fmc and MMC share a short kiss (with the MMC sleeping). But I went exploring everything and deepining anything I could. There are long stints (around 80k words) that the MMC and FMC only has one romantic interaction and it's their only moment at all during this part.

But I don't call it a slow burn because their "get together" arc happens in that "book" and it's like on first third of story... And the first book is mostly focused on MMC where I made the romance area all about frustrate people on his part.