Okay, I went to grad school in Austin, Texas for my education, not because my favorite game publisher was there. :-) But I didn't fail to notice that fact, and I made contact with Steve Jackson Games right away. Before long I was one of their playtesters, taking at look at a variety of GURPS supplements. One of those was GURPS Supers, one of the most eagerly anticipated SJG products. Having played a lot of Champions, but preferring the GURPS ruleset for most roleplaying, I definitely wanted in on this. Through this playtesting I was asked to provide the character designs and a little background story for some pregenerated supers to be included in the book. I contributed Flamin' Jane (pictured on the cover of the worldbook), and some others.
That led to a job to design all of the characters in GURPS Wild Cards. In this case, though, design meant character stats only—all of the background text was provided by the worldbook's author, John J. Miller, himself one of the writers of that series of anthologies. That was exciting, and relied on my own reading and study of the books. I'm sorry I didn't get to meet John or any of the other authors, though.
By this time I was into my summer break from grad school, my teaching assistant job taking a hiatus, and no replacement job to be found. That is, until I signed a contract with SJG to write the first supervillain sourcebook for GURPS, to be called Super Scum. (I myself preferred the name GURPS Bad Guys, but it didn't stick.) You'd be hard pressed to get rich in this business, but it saw me through the summer, and was a great experience! I'm a mechanical engineer, so working on a writing project was something new for me. I ripped through the first three-quarters of the characters, but the remainder were slow going, right down to my deadline. And through it all, I struggled to provide enough background text to fill out the facing page. Most other villain books for other RPGs included just a paragraph or two about the characters, but Steve was sure he wanted the GURPS book to have more than that. Out of that struggle came the Battle Variations section, which gave a shorthand rewrite of each character's stats at higher or lower power levels. (I'd seen something similar done with predesigned Car Wars vehicles.) Though I suggested Battle Variations in part to pad my text, in the end I think it was one of the more useful aspects of the book. More useful than more background, I'd think.
I'm not sure how much my experience is still relevant for today's freelance game designers, but I do have one bit of advice: stick to your schedule. Now, I cannot in all honesty claim that I did that myself, and there certainly was a fury of activity right up to my deadline, but I did pretty well. However, due to my proximity to SJ Games and general familiarity with their editorial staff, I discovered that most freelancers aren't so timely. Let me clarify that—most first time freelancers aren't so timely. Ever notice how some authors of RPG materials write lots of books? That may because they're skilled writers, yes, but I guarantee that they are also dependable. RPG publishers have enough to worry about running their business with concerns over cash flow and printing schedules. They need freelancers that deliver on time, as promised. Do that, and you may just get further work. Good luck!