I applied online. The process took a little over two months. I interviewed with Lockheed Martin around the first week of December 2019.
The entire process actually started around August 2019. I was interviewing with other defense companies, but noticed Lockheed Martin was showing up to a career fair. Coming out of university, it's a rarer occasion that Lockheed would be a part of a career fair than Boeing, Raytheon, or Northrop Grumman in the LA area.
Be advised, it's going to be difficult to get any interaction with Lockheed Martin at a career fair without clearance, unless the event is specifically for college grads. You have to be really opportunistic about it. I just gave my resume to one of the recruiters on break and he handed me an invitation to a social Lockheed Martin was hosting later that day (still without any clearance). The event was really great, and I ended up talking with several managers. One of the managers said he'd send my resume over to see what was open. Fast forward a few months later, around late October, and one of the hiring managers sent me an email asking me to apply for a position that was open.
With the Thanksgiving holiday and other interviews I had to conduct, it took a fair amount of time to schedule the interview. The interview itself was very straightforward. I can only speak for college grads, but be prepared to talk about what you did in school, and what you did in your classes. Best be prepared to also describe some of the more team-oriented things you did in the previous five years in college or if you did any work, since it was pretty clear to me that they were really testing me on how great of a team player you were. Ask a lot of questions, since they really like how curious you are.
I got the offer the next day.