Link to Approval Process Screen

When using approval process you may not specify email template.
In this case SF will use the default template for approval processes with the following structure:

In case you want a customise email template you need to create it.
Sometimes, even when you do create your own email template you want to have the link to the approval screen.

I'll demonstrate how it add it with approval process on Case object.

1.Create email template on Case object.
After desinging the template as needed, at the end add component:
<c:CmpApprovalURL objID="{!relatedTo.id}"/>

<messaging:emailTemplate subject="Case onHold Request" recipientType="User" relatedToType="Case">  
 <messaging:htmlEmailBody >  
  You have been requested to approve or reject this case:<br/>  
  {!relatedTo.Subject}<br/>  
  Contact: {!relatedTo.Contact.Name}<br/>  
  Priority: {!relatedTo.Priority}<br/><br/>  

  <c:CmpApprovalURL objID="{!relatedTo.id}"/>  
 </messaging:htmlEmailBody>  
</messaging:emailTemplate>  

2.Component:CmpApprovalURL.
Using String urlStr at class cls_CmpApprovalUrl

<apex:component controller="cls_CmpApprovalURL" access="global">  
 <apex:attribute name="objID" description="Obj ID" type="Id" assignTo="{!caseID}"/>  
 <a href="{!urlStr}">Log-in to approve or reject</a>  
</apex:component> 

3.Class:cls_CmpApprovalURL.
Need 2 variable :
1.Input for the CaseID
2.Output URL

public class cls_CmpApprovalURL  {  
 
 public String caseID {get; set;}  

 public String urlStr{  
  get  {  
   return cls_createApprovalURL.generateApprovalURL(caseID);  
  }}  
}  

4.Class cls_createApprovalURL
I create the function generateApprovalURL in this seperate class since it can be used for any object for any approval process.
It receive object ID, and return the correct URL for it's approval screen.

public class cls_createApprovalURL  {  
   
 public static String generateApprovalURL(String recordID)  {  
  String url='';  
  
  List<ProcessInstanceWorkitem> workItemLst =   
   [  SELECT id  FROM ProcessInstanceWorkitem  
    WHERE processInstance.TargetObjectId=:recordID];   

  if(workItemLst.size() > 0)  {  

   url='https://'+ System.URL.getSalesforceBaseUrl().getHost() +   
    '/p/process/ProcessInstanceWorkitemWizardStageManager?id=' + workItemLst[0].id;      
  }  
  return url;  
 }  
}

5.The result:

In the email received:

The link from the email: