I'm not sure if you're using Photoshop, or Sketch, but here are two handy videos that will show you how to mark layers as assets (from within Photoshop, or Sketch):
PhotoShop - https://www.youtube.com/watch?v=0cVbzq-Q43M
Sketch - https://www.youtube.com/watch?v=o9kOpAaDpQU
Once you've done that, and exported your design to Zeplin, it does a bunch of the heavy lifting for you. You can then go through and create a color palette fairly easily, which is covered in this YouTube video: https://www.youtube.com/watch?v=tHmzAytSgpI
That should be all that you have to do. From that point, your developers should have all measurements, colors, and assets that they'll need once you invite them to the project.
Hope that helps!
p.s. I'm happy to answer any follow up questions you might have.