Hiii, I'll do my best to answer this! For me, I think my greatest tools are self doubt and being dumb. Talking about doubt... Doubting oneself is pretty natural I think? But I crank it up a little for sure, to make sure that the self doubt is pushed away at the correct time. Being dumb is another thing I do quite often? Where everyone can see that the two of them are in love except the two of them. It also goes hand in hand with doubt quite a lot of the time!

Another good idea is to have a story you can rely on besides just the romance between the two of them. For instance, I like to have several side romances going on, as well what their desires are out of life. There's more to life than just romance, yeah? Jobs, family, friends, maybe even things like magic, political intrigue, or gang violence! They can all add to an interesting story. Making the characters need to focus on something else entirely can help with the burn remaining slow.

Those are probably my biggest tips for writing a slow burn! I hope this helped. ^_^