How do I make characters fall in love?

Short answer: it will depend on the characters, the context, the story you’re trying to tell, etc.

Longer answer: among other things, approaches to this will depend on your starting point. If you have two relatively well fleshed out characters with backstories, desires, interests, goals, personalities etc., and you’ve decided they need to fall in love for the story to work, but can’t figure out how, you may have a problem. So for example, if you wrote Protagonist 1 as a headstrong, tough person who has faced the prospect of an arranged marriage their entire life and doesn’t want to be “tied down”, and Protagonist 2 an arrogant Casanova type who thinks they can make anyone fall in love with them but doesn’t love anyone themselves except briefly or superficially, it might stretch reader credulity for them to fall in love with relatively little effort or change in their respective outlooks, or for their outlooks to suddenly change. Unskilled/lazy authors will force characters together in this kind of situation with plot devices like “soulmates” or “destined true love”, or contrived situations, or simply author fiat (“they were both wary of love but neither of them had ever felt this way before!”).

If you have incompatible characters but need them to fall in love for the story to work, then the way to go is probably to go back and change one or both of them significantly, at least to the extent of toning down traits which make them incompatible. For example, if Protagonist #1 has a clear aversion to societal expectations of marriage but not to romance in general, that allows Protagonist #2 to work as their romantic interest if they are also looking for a romantic affair, not socially-acceptable marriage. If you’re starting out with the intention of writing a romance, it’s worth thinking about this when coming up with the characters - even if you’re writing an “enemies/rivals-to-lovers” type story where the characters initially dislike each other and seem to have nothing in common.

If you have characters who are not strongly incompatible and you just can’t think how to get them together, then there are lots of devices used by romance writers for centuries/millennia to make characters take special notice of each other. For example, give them a shared interest or goal which they don’t share with most people around them, a mutual enemy to bond over disliking, emotional needs which they each fulfil for the other, etc.

Something worth thinking about, particularly if this romance isn’t the main point of the story: do you need/want these characters to fall in love if there’s not an obvious reason for them to do so or path to them doing so? Sometimes, especially with subplots/side characters, it’s better to only romantically pair characters if a romance between them seems to arise organically from the characters as written. In other words, rather than deciding characters are going to fall in love because you feel there should be a romance in the story or it’s convenient (e.g. “pairing the spares”, giving side characters a “happy ending”), maybe only pair characters romantically if they seem to have potential romantic chemistry when you write them together, and/or they seem to have things in common or similar.