You can look at this topic that discusses preparation for the cordeo. It's important if you're going to use the cordeo for riding.Preparation For the Cordeo
And this topic on RamenerRamener and ramener into movement
Here's a topic on vertical flexion that is very important because it discusses what you need to be aware of at the back end of the horse as well:Advice please on vertical flexion
Here is a really nice little video on clicker training for flexion. You can easily do this on the cordeo instead of a halter, but eventually, you should do it with a line as well. Clicker Training teaching flexion
Your slow lateral work on the ground is the best way to get your horse set up for collection and in some of the exercises like shoulder in, you will see that your horse will probably offer to bring the head to a vertical position (with a little lateral flexion). All your training in ramener should not be isolated...it should at least be balanced with the lateral work that you'll find in the ground work stickies. Do both, reward both.
In this way you are training the whole horse in a healthy way and not just a head set. If your training has to be imbalanced, then concentrate on the back end of the horse more than the front. Always!