It's easy. All you need are:
- a PhD in orbital mechanics
- the physique of an Olympic athlete
- a distinguished career as a test pilot
- a fluent command of Russian
- a great deal of luck
Let us know how it goes!
Edited to add: Galvanised by uhoh's comments, I actually googled this ("NASA astronaut requirements"). The official list is not entirely dissimilar to my made-up one:
Today, to be considered for an astronaut position, applicants must
meet the following qualifications:
- Be a U.S. citizen
- Possess a master's degree* in a STEM field, including engineering, biological science, physical science, computer science or mathematics,
from an accredited institution.
- Have at least two years of related professional experience obtained after degree completion or at least 1,000 hours
pilot-in-command time on jet aircraft.
- Be able to pass the NASA long-duration flight astronaut physical.
*The master's degree requirement can also be met by:
- Two years (36 semester hours or 54 quarter hours) of work toward a doctoral program in a related science, technology, engineering or math
field.
- A completed Doctor of Medicine or Doctor of Osteopathic Medicine degree.
- Completion (or current enrollment that will result in completion by June 2021) of a nationally recognized test pilot school program.
The US citizenship requirement is obviously NASA-specific. And perhaps Russian is no longer a requirement (although every little helps).
But they left out the most stringent requirement: a great deal of luck -- they received 18,300 applications in 2016. Or perhaps it is enough to be the very best in every category (which would mean that my list is closer to the truth than the official one.)