To enable the "Recurring revenue" feature, go to "Settings" > "Configure pipelines".
For the pipeline of choice, edit the pipeline by clicking on the blue pencil icon located on the header. Flip the switch to the right to enable "Recurring revenue".
You'll be asked to fill out:
Expected customer lifetime (in months): Specify the average lifetime of a contract. This will be used to calculate the deal's lifetime value in case you did not specify a contract end date yet.
Standard price per unit (in your currency): If your monthly/annual/... contracts usually have the same price per period and unit, you can fill it out here to save time. It will be prefilled automatically (and can be changed if needed).
After enabling "Recurring revenue":
You'll see new fields appearing when you edit an Opportunity
As well as an additional MRR graph in the "Insights" dashboard.