Have a look at testdata/ispconfig-solver/, copy and modify them to match your environment, then run the test
Don't change the name of the secret ispconfig-secret in the manifest.
TEST_ZONE_NAME=example.com. make test
If the test was successfull, you can already apply testdata/ispconfig-solver/ispconfig-secret.yaml to your cluster
k -n cert-manager apply -f testdata/ispconfig-solver/ispconfig-secret.yaml
helm install ispconfig-webhook deploy/cert-manager-webhook-ispconfig -n cert-manager
make rendered-manifest.yaml
kubectl apply -n cert-manager -f _out/rendered-manifest.yaml
See examples/ to add a clusterissuer and a first test certificate to your cluster.