In your FastAPI app, use 'pylsat' as middleware. Initialize the 'Pricing' class with 'price_sats' or 'price_fiat' and a 'conversion_func' for fiat. Next, initialize 'L402Validator' with root key, expiry time, invoice generation function, description, and pricing object. Apply in route handlers. For usage example, visit https://api.autobtc.ai/docs or see demo: https://youtu.be/WI5xnJAV_UQ