I have not done duathlons, but I have done triathlons, so I think it will work about the same.
You will have a race number. When you go into the transition area there will be racks to hang your bike. There will be numbers posted on the ends of the racks, so find the rack that corresponds to your race number. The best place on the rack is at the end closest to the main thoroughfare in the t-area b/c you won't have to mess with trying to get through other people's stuff. It's one of the reasons people get to races 1.5 to 2 hours before the start!
Look around to see how others are hanging their bikes. Most hang them from the seat. Notice in what direction the tires most are pointing. That's probably the best way to hang your bike so it's easiest to remove it and head to the mount area.
In front of your front tire you should lay a bright towel - something that you can easily spot. On the towel you will have your gear - helmet, running shoes, sunglasses, running cap, etc. Make sure you're not taking up a lot of space because they expect to fit a good deal of bikes on the racks, and you don't want to crowd anyone.
When you come back from the run, go to your bike. Remove your running gear, put on your cycling gear, unrack your bike, CLIP YOUR HELMET, and jog with your bike to the mount/dismount area. Get on your bike and go. When you return, get off the bike, unclip your helmet AFTER YOU ARE OFF THE BIKE, jog back to your spot, rack your bike, change back into your running gear and go.
Dus and Tris are run without outside assistance, so you can't have anyone in the t-area, nor should there be anyone on the course who is helping you (you'll be time penalized). If you don't know how to change a flat tire, it's good to learn before the race just in case.
Good luck. I hope this helps.
"God gave you your body as a gift, so you should take care of it." - My Mom