It’s a fairly straightforward process
I followed https://eksworkshop.com/ to get my first EKS cluster setup and the right tools installed, including Helm.
I then installed an nginx ingress
helm install stable/nginx-ingress --name rancher-nginx --set rbac.create=true
and finally I followed the Rancher instructions for installation with Helm (https://rancher.com/docs/rancher/v2.x/en/installation/ha/helm-rancher/). In our case I used certificates from a file, ran Rancher in the same namespace as the ingress and had to specify the ingress class via ingress.extraAnnotations. e.g.
helm install rancher-stable/rancher --name rancher --set hostname=rancher.example.com --set ingress.tls.source=secret --set ingress.extraAnnotations."kubernetes\.io/ingress\.class"=nginx