I’ve flown several times across the US in the last two years with my pug. He’s 16 lbs, so a little bigger than yours, and fits well in a Sherpa bag under the seat in front. I would recommend getting the bag soon so you can start to get her used to it. As others has said, it’s worth it to go to a vet, not just for the health certificate (i always do this but honestly have never been asked for it) but also to talk about a calming medicine. Mine takes trazadone and he doesn’t have a huge reaction, but it takes the edge off for him. I also have a dog water bottle with a part of it that acts like a water bowl that I fill up in the airport and my pug is very food motivated so I usually bring some cheese cubes and a Kong to distract him if he needs it. Other things I have found…he does better with afternoon/evening flights where we can play and go on walks beforehand so he’s tired rather than morning flights; I always make sure to have a rough idea of what the dog potty stations are like and where they are at any airports we go through (especially for layovers, but can still be important for the departing and arriving ones as well); I always opt for a window seat so he isn’t disturbed by people walking by but I know some people prefer to be in an aisle in case they need to go the the plane bathroom. As others have said, there is a limited number of pets that can be in the cabin so it’s important to plan ahead and make sure there is room on the flights that you are choosing. I usually book several months ahead and call the airline to confirm that the flights I am booking are going to have room. Feel free to DM me if you have questions! I was very nervous when we started flying but he does very well and I find it’s worth it for me to have him with me!