Hello LeMecDePhalanx,

Well, that's not an easy question you're asking here and I'm guessing that each Game Designer might have a different answer.  I'll tell you what I did, but please keep in mind that this isn't the only way to get into the industry, just what worked for me.  Feel free to experiment and cherry pick what you think suits you best.

First, I made sure to have as much "technical" design knowledge as I could: I read a ton of books about Game Design (I can't recommand enough "The Art of Game Design" from Jesse Schell, "Rules of Play" from Salen & Simmerman and "A Theory of Fun" from Ralph Koster), I went on https://www.gamasutra.com/ every day to read anything that felt like it would relate to game design and I pillfered youtube for knowledge (you might want to check the GDC's, Extra Credits' and Game Maker Toolkit's channels).  All of this should help you get a better understanding of design techniques that will assist you in making a game out of an idea.  From my experience, Game Design schools tends to have incomplete cursus, so you better hedge your bet.

Second, and that's what should help with ideas, I found ways to help me get inspired.  I started looking at a ton of movies with friends I could discuss them with. I educated myself on as many subjects as I could (Crash Course's youtube channel is a treasure mine and so are the PBS' and the Overly Sarcastic Production's youtube channels).  I also put some efforts learning programmation so that I wouldn't be lost when speaking with my colleagues.  Of course, I also played a lot of games from different genre and took the time to know which one were released.  Looking at a feature from one game can sometime give you an idea for another one, you never know :). 

And lastly, I found ways to show my abilities.  Knowledge is good and all, but it's hard to put on a C.V. or a portfolio.  Back then, I participated to a few game jams with classmates so that I'd have a few more projects to show around that weren't mandated by school.  Game jams are so short and the teams so small, you end up with something that reflects your skills a bit more accurately I think (and at worst, it shows you're motivated enough to work a few week-ends on a personal project).  I tried to get involved with my school's student organisation: I setup a few boardgame-related events for other students.  It was nothing big, but it showed that I was wiling to take initiative and that I was organized enough to put together a small event for 10-20 people. Back then, I also wrote a short story and did a few maps for Warcraft 3 and Stealth Bastard so that I could show what I'd be able to bring in terms of narrative and level design.  

It's been a while since I was hired so the criteria of the company might have changed a bit since then, but I think that what Amplitude is looking for in a new hire is still pretty much the same: someone with a good head on his/her/their shoulder.  We have a test for candidates and while I can't tell you what's in it, I'll just say that it's a good idea to brush up on your analytical skill, as well as math and programming lessons. 

And I think that's it, hope that helped :)

Have a nice day and good luck with your studies (and as hard as it is... try to not overwork yourself!)