Uploading files to Amazon S3 using SigV4 API

I am looking to upload images to amazon S3 using the rest api that they provided. I got to know how to calculate the signing key for SigV4 from this document.
This documentation tells you how the request should be signed. But I find it highly confusing as to what should be signed and where should the cannonical request be placed? Should it be placed in a separate header in the request?

Is there a working example/sample to use SigV4 rest api using java?