there is a welting short video there. Also if you have a car or upholstery supply shop near you, go there to get your welting cord. If you send me a private email with your address I will send you samples of what you are looking for it’s like a poly cord it’s perfect for welting, comes in so many different thicknesses.
To prevent puckering around the curves or corners just clip it. if you need an example I will post a picture.