For Custom Controller

//Page reference to your VF Page
PageReference pageRef = Page.TestPage;
Test.setCurrentPage(pageRef);

//Pass necessary parameter
pageRef.getParameters().put('Id',id); 

//init controller 
CustomCtrl objCtrl = new CustomCtrl();

//Call pageRef mymethod
PageReference objPageRef = objCtrl.mymethod();

//Put system asserts
System.assertEquals (null,pageRef);

For Standard Controller

//First create record
Account acc = New Account();
acc.Name = 'Test Account';
INSERT acc;

//Page reference to your VF Page
PageReference pageRef = Page.TestPage;
Test.setCurrentPage(pageRef);

//Pass necessary parameter
pageRef.getParameters().put('Id',acc.id);   

//Pass your object to controller     
ApexPages.StandardController stc = new ApexPages.StandardController(acc);

//Call controller
CustomCtrl objCtrl = new CustomCtrl(stc);

//Call pageRef mymethod
PageReference objPageRef = objCtrl.mymethod();

//Put system asserts
System.assertEquals (null,pageRef);