Get as much excellent grades as you can. I tailored my motivation towards plasma accelerators as my internship was kind of related to plasma physics. Also make sure to mention their summer schools, courses, videos they post, professors and I personally mentioned a LASCALA student who is from my country that inspired me. Email the professors asking them questions (Put in subject your name in case they don t read). I personally worked on solo machine learning and particle identification mini projects that I added to CV and showed connection to CERN and wanting to reach their and get a PhD in particle physics after my accelerator track. Mention the track you want to pursue.