When you arrive in Chicago, you'll walk into the station and turn left. You'll see a waiting room with white curtains either ahead of you or to your right (depending on which gate you arrive at). Walk past that room and then turn right. You'll see double doors with lots and lots and lots of people walking through them. If you get to the escalator, you've gone too far. Go through those double doors. I'd say it's about 30-40 feet from the tracks to the double doors.
Once you go through the double doors, you'll be in a hallway. Just keep going straight. It's about 60 feet. You'll see an Information Desk on your right, and an escalator and the Amtrak Ticket lines on your left. Turn right after the Information Desk and go through another set of doors.
Now, walk straight ahead, toward the bathrooms. You'll see framed doors with frosted windows. That's the entrance to the Metropolitan Lounge. Go in the doors and walk to the desk. They'll ask you for your ticket for the Cardinal and give you a colored "ticket" that gives you access to the lounge should you leave and come back. It will have a time to come back for boarding.
You can then give the redcap (in the room across from the desk) your luggage. They'll also give you a luggage check tag with a time to come back. This does not check your luggage onto the train. This is like a coat check. It allows you to explore the station while waiting.
You can wait in the lounge if you're nervous. They have free beverages, snacks, wi-fi, and televisions. The furniture is comfortable, and it's a nice place to wait for your train. I do suggest exploring the station, though. The Great Hall is absolutely gorgeous and is a must-see. To get there, just walk out the lounge doors, go straight ahead past the escalator, and keep walking straight.
Here is a map:
I put a star by the tracks where the trains from Kansas City arrive. I put another star where you turn right through the double doors, a third star where you turn to head to the lounge, and a star on the lounge.
This is the lounge:
When it's time for the Cardinal to board, the lounge attendant will line everyone up and walk you out to the tracks. You'll head toward your train and look for your car. There will be train attendants at some of the entrances, and they can help you find your car and room. The room numbers are by the doors.