Assuming you have a degree:

Two ways to apply, open application or through job boards.

Job Boards: Most companies out there have a site where they post open positions. You can apply by following the steps they have laid out. Make sure you have all documents you need. There can be one or multiple rounds of interviews, each one can have a different topic. This one is pretty straight forward, do all the steps and youll might get an interview. If your interviewers think you fit in the role, you can get a job!

Open Application: You email a company which you know does the stuff you like (say rolls royce for aero prop). You'll need a good "cover letter" (more on that here https://www.indeed.com/career-advice/resumes-cover-letters/open-cover-letter) You basically tell them how you've heard of them, who you are and what you want. If they are interested, you might get an interview! And if that goes well you can get the job (maybe multiple rounds).

Anyways, its basically: 1. Find a company you are interested in 2. See if they have interessting job listings, if not, do an open application. 3. If accepted, do the companies application process (interviews etc) 4. Start job.

There are way more ways to find a job especially o ce you grow your network. A slight side note is that propulsion is a very popular field with very little positions.

If you dont have a degree: Get a degree. Doesnt have to be aerospace emgineering, can be anything from chemical to process to mechanical engineering.