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Question: How do I create custom error page and utilize it in sh404?

Answer: First create a custom error page as an UNCATEGORIZED article via the article control panel.  Set the section and category to UNCATEGORIZED. Publish it.

In the sh404 control panel > sh404 configuration, set there is a drop down menu.  Your ERROR document should now be an option in the drop down box. Select your ERROR document.

Save.

Note: Though the info icon next to the error field says the published state does NOT matter, we found that it DID matter.  If the document was unpublished, the default Joomla error page showed. 

Note: By default the sh404 system uses the home page item id.  In some cases this can be a problem if you have a special home page modules published.  To resolve this we turned ON the ADVANCED control panel of sh404.  In the ADVANCED control panel we modified this line:

// if not 0, will be used instead of Homepage itemid to display 404 error page
$shDefaultParams['sh404SEF_PAGE_NOT_FOUND_FORCED_ITEMID'] = 0;

We changed the 0 to the article ID, which you can find via the articles control panel.

$shDefaultParams['sh404SEF_PAGE_NOT_FOUND_FORCED_ITEMID'] = YOUR ERROR PAGE ID NUMBER;

Note: We also discovered that the SEF url of error page remains whatever URL caused the problem. At the moment, we are undecided as to whether we like this or not.