Make a plan and keep it under wraps. Anyone involved needs to know how important it is to you and her that she doesn’t find out your plan. Let them know part of the excitement for you both is the surprise.
Find a way to set the date and time and other arrangements without her finding out. Now you have to keep it a secret and have a plan on top of it so you can get her to let it play out. For example, if you have a destination for it to take place, you need a solid reason to get her their without her catching on. It must be rock solid reason that she can’t say no, or she may get tipped off. You can find a way to slide it into activities you would be doing anyway, like if you had wanted to propose at a park you first met, you could have a friend of hers arrange to meet the friend for a dog walk, and surprise her when you are there. The point being, you need to make your plan of proposing but, on top of that have a layer of the story that she will follow, not knowing you have a much deeper plan to surprise her.