So you got your first gig--congrats! It probably feels a little surreal. You've loved comics your whole life and fought tooth and nail for this moment and you made it happen. You got your first page rate, and damn is it low, but that's okay cause you're in the biz, kid! Enjoy the moment, and if you're talented, pleasant, organized, and a little bit lucky you'll have many more moments like it.
Okay, you've lettered your first comic and are now getting acquainted with the editorial and proofreading process. Hopefully since this is your first gig they aren't re-writing the entire book on you, but it happens (and don't worry, we'll eventually talk about that BS here). You manage to fix all the mistakes, and yep, there are bound to be some. And yes, even you will have made mistakes on your first book. That's okay, I still make mistakes, too. Just try to improve every time. There's a thin line between mistakes and messiness and your editor will let you know if it's the latter. Anyway, so you've delivered your final files and the job is done. The payment is on the way. Now what?
Now, you should be leveraging this gig into as many gigs as you want to take on. Start with your editor or publisher you just delivered your first work to. If they're happy with your work and you in general they will use you again. The thing about comics is that they always need to be lettered! But Steve, how?! Well, just ask. Ask your editor if they have additional work you can take on. If not, do they know of any other editors in need of lettering? No? Don't fret, don't be annoying about it, and as soon as you can show the world your first book. Post it to your socials, share it with your friends. Talk about your positive experience with the work and how much you'd like to do it again. Tell people you're looking for work. Reach out to other creatives, other letterers and ask if they know someone looking for lettering (or whatever it is you do). Leverage this work in all the ways you can, while being sincere, non-spammy, pleasant, and professional.
And that really should be enough to turn one gig into another. Rinse and repeat.