The short answer is there is no short answer, and the long answer depends very much on the exact model of Simon XT controller that you have.
You can take a look at the project reports on the quick browse lists in the community – created wiki for the security projects and you’ll see what some other people have done.
Just be aware that anything with the SmartThings thingshield device has to be done a different way now because that device was discontinued. But there are a lot of possibilities.
I’ve moved this thread to projects so that you can get an individualized responses based on your own needs and set up. People tend to come to this kind of integration with one of two perspectives:
they already have a lot invested in existing sensors which are not able to be paired directly to the smartthings hub, and they want to be able to reuse those sensors but run the logic and rules from the SmartThings side or
they still want to use their old security system as their primary security, but they want to add some additional integration while using SmartThings for their home automation. In this case, it’s sometimes just a matter of adding the ability to arm/disarm the security system from the SmartThings side, or it may go a little deeper.
The middleware, additional devices, and code required for those two options are very different, so if you can tell us a little bit more about what you’re trying to accomplish we might be able to help better.