If you open the spoiler tab I included some images of some assisted hatches that I did. They aren't your exact situation but they will make it easier to show you kind of what to expect to see inside the egg. First I candle the egg and mark the air cell. I use a screw sterilized with rubbing alcohol to carefully score and pick at the egg to open a hole in the air cell. I'm careful not to push because you can see how close the chick can be to the shell even when it looks like they're further away when candling.
Ok, so I want to go through the air cell.... Once I'm through, I should see the white, papery membrane. Watch to see if it is moving? If no movement, it is likely DIS. If movement is detected, I can apply ointment which will make the membrane translucent so I can see if there are any veins (not ready to hatch) or no veins (ready to hatch)? Then what?