It depends. For the most part, I write chronologically, but if there is a particular scene that is standing out in my mind, I might just write that one first! In fact, for the second long!fic I ever wrote, the first chapter I wrote was actually the epilogue. Sometimes I'll also skip scenes or parts of scenes if I'm feeling blocked on them and it's holding up the writing. But I do generally prefer to write chronologically so that I have a good grasp of the way feelings etc. are evolving. Because I do end up adding things I'd not planned in the outline, if I skip around too much I find it's harder for me to ~feel~ where the characters are at, because I've not yet written their development, if that makes sense? But everybody is different!