The way to collection remains the same, bit, bitless or cordeo. Just follow the Gymnasium exercises the old masters taught us.
I have to say that I only add the cordeo when the horse has reached self carriage through Gymnasium play and the work in hand with a soft cavesson and later Gymnasium Riding with the soft cavesson after which the cordeo takes over slowly.
The tradition down is all in the seat. If you want to go forward, rais yourself a tiny bit by squeezing buttocks and finding pressure in the stirrups and move with the horse in the specific gait.
Then if you want to transition down, release your muscles tension, breath out and lean a tiny bit forward with your upper body.
The key to all your do is your timing of reward. It is essential to reward the precise moment the horse does what you have asked or leans towards what your are asking (slowing downs instead of halting of you are learning this with your horse as a new exercise). You don't need a clicker for that, I don't use one at all
The second thing is that the horse never encounteres punishment. This way the horse will never be scared of giving 'wrong answers'. This will keep the horse experimenting and the new exercises will come of their own accord.