You can grab some neon mason line at pretty much any hardware store. In Home Depot and Lowes, it’s by the giant spools of rope and chains.
You can make this one by using four pieces of string (two that you won’t see). Alternate sides, making one basic hitch knot. To make it look more intricate, you can push the knots way close together, or you don’t have to at all.
Otherwise you can just tie the ends together! Because mason line is made of nylon, knots come out really easily. That’s where a lighter comes in. Hold it up to the knot and right when it starts to melt the cord, push the bottom of the lighter down on it so it melts onto itself.