Obviously clone the repo and yarn install (or npm if you prefer).
Create a Stripe account and define some products with prices - notice the 'API ID' stored against each product price.
Create a https://graphcms.com/ account. Define products in the schema to match the query in services/graphcms/get-all-products.ts. Create the same products you added to Stripe setting the stripePriceId to the product/price-api-id copied from stripe eg "price_1H5UA8Gr1aMYF0A8n2xKu8wW".
Copy .env-example to .env.local and set the values for Stripe and GraphCms
yarn dev or npm run dev