Drilling for Engine Mount

Ready to drill the firewall for the engine mount and have read the plans and studied the print but there is not a lot of direction.

This seems like it would be easy to mess up so I wanted to ask if there are any "gotcha's" to look out for or special processes that are not covered in the plans that people have done. Really would hate to have to disassemble the structure due to a bone head mistake.

Thanks,