My husband makes a gentle leader just out of a leash in a pinch. With leash on dog. You just make like a ribbon crossover (think pink breast cancer ribbon) and holding it then twist your wrist (essentially facing the ribbon the opposite way it was originally facing) ...and slip it over the snout. It is called a half hitch. If it falls off you have twisted your wrist the wrong way - try again the other way. It works the same for our dogs as the gentle leader and has helped a few times when I havent had the leader with me.
Remember you cant really do it wrong, it wont stay on the snout, so you will know if you made a successful half hitch. This leader will tighten like a choke chain on the snout so dont pull hard on it when you are walking (Abby wont pull anyways because she doesnt like the sensation of it tightening up) It came in handy when we were waiting for the ferry, I let her out to pee and didnt bother suiting her up with the real gentle leader, kids with their dog came (from nowhere) to where I was and she was pulling like crazy, 1 second later a flick of my wrist, over her snout and no pull.