How do pilots get their call signs?

Naval Aviators are 'assigned' them by their peers in a tongue-in-cheek ceremony. Often to their chagrin.

I have a friend who's Callsign is ratface. Because when he says right face his Alabama accent forces him to pronounce it "rat..... FACE!"

The use of personal call signs outside of the flying environment came after I left the Air Force in 1991. It was and probably is most prevalent in the fighter pilot environment. The movie "Top Gun" popularized the practice.

As an airlift pilot, we used two sets of call signs when flying. The dynamically assigned call signs were selected from a list and assigned to the mission. Usually a whole flying Squadron used the call sign followed by two or more numbers normally based on a mission designator. This made for some interesting choices such as tug, truck, turgeon, speedy, sanctity etc. It was the copilot's job to keep track of the call sign. The other call sign was the generic "Air Force" with at least the last four digits of the aircraft tail number, such as "Air Force 1861" The FAA and most foreign air traffic control systems were okay with either method of identifying a flight.

The airlines use a word or the name of the airline along with a route designator. For example "Speedbird 1" was the British Air flight from New York to London. It's common to hear "United Flight 257", or "Southwest 098" on the radios. Civilian aircraft in the US use their "November" number or aircraft type and registration number. "November 79-1568" or "Piper 63-0942".

When I was a pilot training instructor, every IP and Student was assigned a call sign. The call sign was composed of a word based on where the officer was assigned, Wing, T-37 Squadron, Student Squadron, etc. As a Wing Safety Officer, I was assigned "Tweet". The number was assigned from a list allocated to the organization, so I was "Tweet 41." The Wing Commander was Tweet 01. Student pilots at Columbus AFB were "Bam" with a number.

Fighter and Attack Squadrons may have more latitude in assigning call signs these days, so they can become nicknames for the pilots. Outside of the flying environment they can be used to impersonally refer to a person to give credit or deflect blame for an action. "Speedy" over there missed the target nine times today. "Bald Eagle 1 has a few words to say about his intercept of the Russian bombers."