The first step to optimize the critical rendering path is to:

• minimize number of critical resources: eliminate them, defer their download, mark them as async, and so on
• optimize the order in which the remaining critical resources are loaded: download all critical assets as early as possible to shorten the critical path length
• analyze and characterize your critical path: number of resources, bytes, length
• optimize the number of critical bytes to reduce the download time (number of roundtrips)