How to verify XML Signature manually without using API?

Question

I have an XML Signature file like this :

<?xml version="1.0" encoding="UTF-8"?>
<PurchaseOrder>
 <Item number="130046593231">
  <Description>Video Game</Description>
  <Price>10.29</Price>
 </Item>
 <Buyer id="8492340">
  <Name>My Name</Name>
  <Address>
   <Street>One Network Drive</Street>
   <Town>Burlington</Town>
   <State>MA</State>
   <Country>United States</Country>
   <PostalCode>01803</PostalCode>
  </Address>
 </Buyer>
 <Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
  <SignedInfo>
   <CanonicalizationMethod
    Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315"/>
   <SignatureMethod
    Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
   <Reference URI="">
    <Transforms>
     <Transform
      Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
    </Transforms>
    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
    <DigestValue>tVicGh6V+8cHbVYFIU91o5+L3OQ=</DigestValue>
   </Reference>
  </SignedInfo>
  <SignatureValue>
   dJDHiGQMaKN8iPuWApAL57eVnxz2BQtyujwfPSgE7HyKoxYtoRB97ocxZ
   8ZU440wHtE39ZwRGIjvwor3WfURxnIgnI1CChMXXwoGpHH//Zc0z4ejaz
   DuCNEq4Mm4OUVTiEVuwcWAOMkfDHaM82awYQiOGcwMbZe38UX0oPJ2DOE=
  </SignatureValue>
  <KeyInfo>
   <X509Data>
    <X509SubjectName>
     CN=My Name,O=Test Certificates Inc.,C=US
    </X509SubjectName>
    <X509Certificate>
     MIIB9zCCAWCgAwIBAgIERZwdkzANBgkqhkiG9w0BAQUFADBAMQswCQYD
     VQQGEwJVUzEfMB0GA1UEChMWVGVzdCBDZXJ0aWZpY2F0ZXMgSW5jLjEQ
     MA4GA1UEAxMHTXkgTmFtZTAeFw0wNzAxMDMyMTE4MTFaFw0zMTA4MjUy
     ...
    </X509Certificate>
   </X509Data>
  </KeyInfo>
 </Signature>
</PurchaseOrder>

I want to verify the Signature manually without using The XML Signature API.

I know how to recalculate in JAVA code the Digest Value for comparing with the value stored in DigestValue Element.

Now I need to complete the next steps for verifying the Signature validity. I found in this article 3.2.2 Signature Validation on how to do what I want but I don't understand any thing in step 2.

Could anyone help explain step 2 in Java.