Create aws_secret_access_key for S3 bucket

Hi all, I'm very new to this, and got very lost in the docs, so likely what I want is "obvious", but I'm afraid I must beg help with it, or at least with finding a document that describes the path I should take. See the tl;dr below if you don't think the background is helpful.

background-------------------

In the past, I have used an S3 bucket created and owned by another company. I uploaded data to it using the "aws s3 sync" command line tool. This did a good job of retries (important since in a typical 200GB upload from a consumer-grade ISP connection, some things would inevitably fail and retries are important.

As part of this, I the company had given me an aws_access_key_id and aws_secret_access_key to make the connection.

Fast forward to the present day. I created a bucket of my own, and started copying data into it using the web page tool s3.console.aws.amazon.com/s3/<myBucket>/upload... this works tolerably well, but doesn't do the "sync" like operation, which leaves me trawling through a couple of hundred files in a directory structure trying to work out what needs a retry.

tl;dr ----------------------------

So, to my actual goal (I think). I need to access this using the "aws s3 sync:" command line tool, but for that I need to create a key. I cannot see how to do this. Can someone help, please?
TIA
Toby