The first step in trying to make your ex a friend is to stop talking about feelings.
The next step is to avoid talking to each other in the same way that a couple who are dating would.
A way that you can go about achieving these steps is to be fully aware of when your conversation has veered into the sentimental.
Make sure that you stop each other at that moment and veer your conversation back to a platonic subject.
You have to both pay very close attention to your conversations and make it a point to correct course whenever one or both of you starts becoming sentimental.
The more proactive that you both are in doing this, the easier it will get.
Something else that you can both do is start making an effort to talk to and possibly go out on dates with other people.
The sooner you start allowing new potential romantic prospects into your lives, the easier it will be for the both of you to develop boundaries.
You would both feel awkward with talking about feelings when there are other people that you are going out on dates with and sharing intimate conversations with.
If you both start going out with new romantic prospects, you will just have a natural inclination to want to be more private with certain aspects of your life.
This will often keep the both of you from talking about feelings or talking to each other in the same way that a couple who are dating would.