Serving resellers is quite similar to serving hosting customers.
If you use Plesk without any automation solution, then to start serving resellers, you should:
Create a reseller plan by running the Service Plans > Reseller Plans > Add Plan wizard in the Server Administration Panel or clone an existing reseller plan by clicking Clone Plans.
Define plan properties. Learn more about reseller plan properties in Appendix B: Properties of Reseller Plans and Subscriptions.
When a reseller is ready to subscribe to the plan, create the reseller account and subscription. Learn more in the section Subscribing Resellers to Plans.