I am not sure what do you need help with?
You may find notes about adding nodes here: [url]Redirecting
Most important thing in this case may be network latency from the current DC to AWS - please note that it will affect transaction commit response times, as Galera needs new writes acknowledged from all the cluster members. Another problem may be if WAN link is unstable - it may disrupt the existing cluster as well. So it’s best to test the link first before proceeding.
Please also check:
[url]Percona Live Europe Amsterdam 2015
[url]https://www.percona.com/blog/2017/02/01/wan-synchronous-clusters-deal-with-latency-using-concurrency/[/url]