First of all, stay calm. Like birds, dinosaurs have extremely sensitive motion detection. Crouch behind a cabinet or trunk and stay absolutely motionless. Soon the dinosaur in your attic will think you have magically disappeared and it will wander over to the dormer window. Next, find the mannequin with the wedding dress on it. Most attics have a mannequin with a wedding dress on it. When you have spotted it, run toward it and stand behind it. Then stick your arms in front of the manneqin and wave and say "Hey, I'm over here and I'm wearing a wedding dress." The dinosaur will run at you and strike the belly. The mannequin's belly. You are saved. Run away before the dinosaur realizes that there is no flesh underneath the wedding dress.
If you're lucky, the live dinosaur in your attic will be a vegetarian. If this is the case, create a trail of lettuce heads that leads to the nearest highway. A bus or large truck should take care of your dino-pest.