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You can add a RenderMan RIS ROP from the RenderMan Shelf or from the out pane.
See https://www.sidefx.com/docs/houdini15.5/nodes/out/ris for RenderMan RIS ROP parameter description.
For more on RenderMan options, see Rendering for more on sampling, filters, etc.
Using RenderMan Shelf
We can use the ROP tool 
in the RenderMan Shelf to create a ROP that automatically adds an intergrator VOP to your RIS Network.
Using out pane
Add RenderMAN RIS ROP
Add Integrator
In your RIS Network, add an integrator VOP inside your RIS Network, e.g. PxrPathTracer
Set the Integrator
Simply drag the path of your integrator VOP path to the Integrator in RenderMan RIS ROP. If you skip this step, by default, it will use the PxrPathTracer integrator with its default parameters.