Exploration CGs don't have a separate "hand in the exploration data here to contribute to the CG" button. They're like trading CGs. You sign up for the CG, then any explo data you sell at the UC office at that station will automatically contribute towards the CG.
The CG counter, however, does not count credits earned, or take into account the quality of the data. Unless they've changed things for the latest CG, you get one submission point per system scanned. It does not matter what object in that system is scanned, whether it's Sag A itself, or a normal star, or an Earth-like, or an ice moon, or an asteroid cluster. 1 scan = 1 point. But multiple objects do not count; if you scan 150 stars, planets and moons in a large star system, you still only get 1 point for that system.
It does not have to be "real" exploration data, either. You can scan systems in the bubble, and those scans still count towards your CG total.
So "Exploration CGs" really have very little to with Exploration as it is usually practiced in-game, but an Exploration CG does offer a nice little bonus cheque if you happen to be returning to the bubble, or are not far away from it, when one is running. Just be aware: if you're rushing back to the bubble to contribute to the CG, there may be hundreds of CMDRs doing the same. If a whole bunch of explorers all arrive back at once and cash in their huge stockpiles of explo data all at once, then an Explo CG can advance through the tiers pretty quick and even end much earlier than anticipated, leaving the latecomers rushing back for nothing.