A really good halt halt comes with impeccable timing. There is NO point of applying a half-halt if your horse’s hind leg is on the ground and unable to move when you ask it to move. Good riders know how to feel and ask for their half-halt at the right time to be effective.
At a very basic level your trot to walk or trot to halt is going to happen on the down phase of your posting trot. This means you half-halt with your outside shoulder and hand and in those moments before the transition when you are sitting in your trot.
To ask your horse to trot from the canter you ask on the down phase of the canter. Your canter has three phases that are up, down, and then slide. If you want a gorgeous canter to trot transition in front of the judge in your training level dressage test you need to apply the halt halt during the down phase of your canter.
If you are schooling second level and want to start your simple changes you have to apply the half-halt on the UP phase of your canter to achieve the perfect timing for canter-walk transition. It's not an all at once and hold thing either, it's a 2 or 3 strides out start asking and releasing in the UP phase only and by the third time you have transitioned.
So next time you ride your horse instead of thinking about OMG i’m stopping...think more like a dressage rider and be specific about when and how much pressure you use to stop.