This will depend on the cat harness you buy- if you have an “H” style cat harness, you’ll need to find a decent fit to begin with, so be sure to check the size before you buy. Your cat will need to either step inside this harness or, more likely, you’ll have to have the fiddly task of putting the harness around your cat. Similar to a collar, you can choose to loosely place this over their neck and chest, and tighten as required, or completely undo the “H” harness to reattach it around the correct areas - do what is best for your and your cat.
The cat harness vest will usually be placed over your cat’s head, much like a t-shirt. Again, this can cause some distress if your cat isn’t used to it at first but they’ll soon understand that this movement leads to their freedom, so will allow it after a while. Vest harnesses also come in the form of a waistcoat-style opening at the front, which may be easier for you but can lead to an increased likelihood of escape.