For my local CAT Scale:
1. Pull on to the scale in the correct direction (signs posted). Stop with your front axle on the first pad, and your rear axle on the middle pad. You will have nothing on the third pad unless you are pulling a trailer.
2. If you cannot reach, get out and then push the call button. It will be positioned so that it can be reached by the cab of a semi.
3. The attendant will ask for your truck number. Just say "private", or give them a number, it doesn't matter. This is just to identify the semi being weighed. Ever notice the numbers on the hood of semis?
4. The attendant will tell you that you are good, and you can pull off and go park. Walk inside to the counter and get your weight slip for your number/private, and pay.
Weights will be displayed by axle (Steer, Drive, Trailer).
You may benefit at taking a quick walk by the scale to see the 3 pads so that you know when to stop, but the front of your rig will be near the exit end of the scale.