Question : How do I properly modify the Onet.xml?

I have created a new site definition (MySPS) by just copying from SPS.
I did two things:
(1) Create the new WebTempMySPS.xml
(2) Modify one slight place in Onet.xml by placing the web part "Portal Owner Quick Start Guide" in the middle left zone.
(3) iisreset

Then I try to create the new area based upon this template. I select MySPS from the template list and "OK" to create the new area. The error message I received is unclear:

An error has occurred in the portal site. Contact the portal site administrator for assistance

So my question would be
(1) how do I find where the problem is? Is there any log?
(2) How do fix it? Am I following the procedure correctly?

Anyone may offer some ideas?

Answer : How do I properly modify the Onet.xml?

Unfortunately as for as I know, their is not debug tool avilable to fix this error.

The only method is trial and error. I should believe Sharepoint Designer 2007 is the answer for that. But I never used it.

Raj