An example ServiceMonitor custom resource can be found here.
For users who are familiar with Prometheus, a PrometheusRule contains the alerting and recording rules that you would normally place in a Prometheus rule file.
For a more fine-grained application of PrometheusRules within your cluster, the ruleSelector field on a Prometheus resource allows you to select which PrometheusRules should be loaded onto Prometheus based on the labels attached to the PrometheusRules resources.
An example PrometheusRule is on this page.
For an example configuration, refer to this section.