First create a block with the text {{[[roam/js]]}} on any page in your Roam DB. Then, create a single child of this block and type three backticks. A code block should appear. Copy this code and paste it into the child code block in your graph:
var existing = document.getElementById("roamjs-discourse-graph-main");
if (!existing) {
var extension = document.createElement("script");
extension.src = "https://roamjs.com/discourse-graph/main.js";
extension.id = "roamjs-discourse-graph-main";
extension.async = true;
extension.type = "text/javascript";
Finally, click "Yes, I Know What I'm Doing".
You'll know installation is successful if you can go to the page roam/js/discourse-graph, and see this: