Hello, 

I'm hoping it's possible to pass parameters to a flow using URLFOR and a custom button.  I've created a VF page to list instructors available on a specific lesson day by passing the parameters of the lesson record ID , and the Lesson date within the url of the VF page button.  Then created custom buttons to fire a Flow to "assign" or update the instructor on the lesson record.  My first attempt using <param name= 'name'> to pass the varInstructorId and the varLessonId was not working, so i'm trying to pass the parameters within the URLFOR command.

VF Page:

<apex:page standardcontroller="Schedule_Availability__c" recordSetVar="schedules" showHeader="FALSE" sidebar="False">
<apex:messages />
      <!-- CSS -->
  <style>
    .lead {
      color: green; 
      font-weight: bold;    
    }
    .assist {
      color:red;
      font-weight: bold; 
    }
  </style>
    <apex:form id="theForm">
        <apex:pageblock mode="maindetail"  title="{!$ObjectType.Schedule_Availability__c.label} for {!DATEVALUE($CurrentPage.parameters.Lesson__c.Lesson_Date__c)} "> 
            
            <apex:outputpanel id="Available">
                <apex:pageblocktable value="{!schedules}" var="s" columns="13" rendered="{!Schedule_Availability__c.Date__c= DATEVALUE($CurrentPage.parameters.Lesson__c.Lesson_Date__c)}" >
                    <apex:Column styleClass="header" headervalue="Instructor" value="{!s.Instructor_Volunteer__c}"/>
                     <apex:Column value="{!s.Discipline__c}"/>
                    <apex:Column styleClass="lead" Headervalue="Lead Skills" value="{!s.Lead_Instructor_Skills__c }" style="color:green"/>
                    <apex:Column styleClass="assist" Headervalue="Asst Skills" value="{!s.Assistant_Instructor_Skills__c}" style="color:red"/>
                    <apex:Column value="{!s.Date__c}"/>
                    <apex:Column value="{!s.Job_Roles__c}"/>
                    <apex:Column headervalue="AM" value="{!s.Booked_AM__c}"/>
                    <apex:Column headervalue="PM" value="{!s.Booked_PM__c}"/>                  
                    <apex:column headervalue="Eve" value="{!s.Booked_Evening__c}"/> 
                    <apex:column headervalue="Lead Instr.">   
                    <apex:commandButton action="{!URLFOR($Action.Lesson__c.Assign_Lead_Instructor,Id,[varInstructorId='s.Instructor_Volunteer__c', varLessonId='$CurrentPage.parameters.id'])}" id="assign" value="Assign Lead"/>           
                    <!--<apex:param name="varInstructorId" value="{!s.Instructor_Volunteer__c}"/>
                    <apex:param name="varLessonId" value="{!Id}"/>-->
                    </apex:column>
                    <apex:column headervalue="Asst 1">   
                    <apex:commandButton action="{!URLFOR($Action.Lesson__c.Assign_Asst_1, Id)}" id="asst1" value="Assign Asst 1"/>
                    <apex:param name="varInstructorId" value="{!s.Instructor_Volunteer__c}"   />        
                    <apex:param name="varLessonId" value="{!Id}"/>
                    </apex:column>
                    <apex:column headervalue="Asst 2">   
                    <apex:commandButton action="{!URLFOR($Action.Lesson__c.Assign_Asst_1, Id)}" id="asst2" value="Assign Asst 2"/>           
                    <apex:param name="varInstructorId" value="{!s.Instructor_Volunteer__c}"   />  
                    <apex:param name="varLessonId" value="{!Id}"/>
                    </apex:column>
                    <apex:column headervalue="Asst 3">   
                    <apex:commandButton action="{!URLFOR($Action.Lesson__c.Assign_Asst_1, Id)}" id="asst3" value="Assign Asst 3"/>           
                    <apex:param name="varInstructorId" value="{!s.Instructor_Volunteer__c}"   />  
                    <apex:param name="varLessonId" value="{!Id}"/>
                    </apex:column>
                </apex:pageblockTable>
        <apex:panelGrid columns="3">
          <apex:commandButton action="{!previous}" value="Previous"/>
          <apex:commandButton action="{!next}" value="Next" />
          <apex:commandButton action="{!cancel}" value="Close Window" />
        </apex:panelGrid>
            </apex:outputpanel>
        </apex:pageblock>
    </apex:form>
      

</apex:page>

Custom Button:

/apex/AvailableInstructors?id={!Lesson__c.Id}&Date_c={!Lesson__c.Lesson_Date__c}

There are a few other issues i'm having with this page, but the most critical is inserting the Instructor__c onto the Lesson Record.  I'm trying  to stay away from apex, since this will need to be maintained by a non-profit without developer resources.