You can create a new cluster by issuing:

cat <<EOF | kubectl apply -f-
apiVersion: mysql.presslabs.org/v1alpha1
kind: MysqlCluster
metadata:
  name: my-cluster
spec:
  replicas: 1
  secretName: my-cluster-secret
---
apiVersion: v1
kind: Secret
metadata:
  name: my-cluster-secret
type: Opaque
data:
  ROOT_PASSWORD: $(echo -n "not-so-secure" | base64)
EOF