char password[] = "test".toCharArray();
File file = new File("test.pfx");
KeyStore keystore = KeyStore.getInstance("PKCS12");
FileInputStream fis = new FileInputStream(file);
keystore.load(fis, password);
fis.close();

// extracting private key and certificate
String alias = "xyz"; // alias of the keystore entry
Key key = keystore.getKey(alias, password);
X509Certificate x509 = (X509Certificate)keystore.getCertificate(alias);

// filling the SignatureConfig entries (minimum fields, more options are available ...)
SignatureConfig signatureConfig = new SignatureConfig();
signatureConfig.setKey(keyPair.getPrivate());
signatureConfig.setSigningCertificateChain(Collections.singletonList(x509));
OPCPackage pkg = OPCPackage.open(..., PackageAccess.READ_WRITE);
signatureConfig.setOpcPackage(pkg);

// adding the signature document to the package
SignatureInfo si = new SignatureInfo();
si.setSignatureConfig(signatureConfig);
si.confirmSignature();

sigPartName = PackagingURIHelper.createPartName("/_xmlsignatures/sig1.xml");

SignatureInfo si = new SignatureInfo();
si.setSignatureConfig(signatureConfig);
Iterator<?> iterator = si.getSignatureParts().iterator();
int i = 1;
while (iterator.hasNext()) {
   iterator.next();
   i++;
 }
sigPartName = PackagingURIHelper.createPartName("/_xmlsignatures/sig" + i + ".xml");