First time using photo etched parts. Some questions

I am building the Italeri MB Actos MP4. I ordered photo etched parts from CTM. I'm wondering:

1. How to attach the parts? For example: The 5th wheel rails. Do I just glue them on over the Italeri part then paint?

2. Are any of them replacement parts (as opposed to putting them over existing parts)?

3. What glue? Thinking of white glue (like elmers).

Thanks!