[SalesForce] How to set flow variables from Process builder

In the process builder you need use reference as follows:

Sample flow looks like this:

This flow, takes the id from parameter and updates an checkbox to true of the Contact record.

Here is a quick and dirty example of an implementation via an Action on the Quote Object:

When the checkbox is changed, you can use a PB to update the related Opportunity.

To create an action, navigate to the Object Manager, choose Buttons, Links and Actions, and create a new Action. Choose to update a record, and select the checkbox field in the Page Layout. In the 'Predefined Field Values' section, set your Quote status to 'Accepted'.

Repeat the steps for a 'Quote Denied' action.