Sounds like my first time!
You will get it!
For layered effects I always wait atleast 3 hours between pouring layers.
So I would mix up enough resin for only the first layer then pour it.
Come back 3 hours later mix only enough resin to cover the embed then add the embed.
Then 3 hours later I would come back mix my 3rd and final batch of resin and mix the glitter in with it then pour it.
So in short only mix up what you need right then and there. Because it sits up in the cup just like it does in the molds.
I can't wait to see what you made!