It is explained in detail in istio documentation under VirtualService.

There is also nice simple explanation and example here:

Canary Deployments

A canary deployment is a strategy for safely rolling out a new version of a service. With Istio, you can use percentage-based traffic splitting to direct a small amount of traffic to the new version. Then you can run a canary analysis on v2 (like check latency and error rate), and finally direct more traffic at the new version until it's serving all traffic.

apiVersion: networking.istio.io/v1alpha3
kind: VirtualService
metadata:
  name: helloworld
spec:
  hosts:
    - helloworld
  http:
  - route:
    - destination:
        host: helloworld
        subset: v1
      weight: 90
    - destination:
        host: helloworld
        subset: v2
      weight: 10

apiVersion: networking.istio.io/v1alpha3
kind: DestinationRule
metadata:
  name: helloworld
spec:
  host: helloworld
  subsets:
  - name: v1
    labels:
      version: v1
  - name: v2
    labels:
      version: v2

UPDATE:

Session affinity aka sticky session can be added with DestinationRule using hashed based loadbalancing according to istio documentation.

Hope it helps.