Sure, you could go to the store and buy letters for a garland, but what if you want a specific font or design? Also, what if you don't want to pay a lot of money? Well, it might be worth considering making your own, using Canva and a printer. Just make your design in Canva and print it out on letter stock or photo paper. Normal paper doesn't look as great.
You can attach everything to your string using clothespins. It also isn't a bad idea to add some ribbon to each side like in they did in this photo.