When system is very complex at back hand, you need to design a interface which makes it simple with user point of view and need to make sure that the interface provided by you provides continuous flow to users to finish that complete process.

Application like shopping cart design which display product feeds from different websites, railway booking tickets form different airways are ideal example for this. Here is basic but very importance tips for designing interface for complex process.

Divide the process in different sub module

Design a module that way each needs input from previous one to process

test the result of each module and optimize it

After finishing the all modules, view them as one process and test with yourself as normal users and identify the problems and solve it intimidate.

this way you will be able to design a interface for users with part of different pages to deliver complex system in simple way.