Basic HiJacking Tutorial - @ThisIsGame0ver

Basic Account HiJacking Tutorial -

Go to google and search (with quotes) "designed by phpvalley"

You'll get a ton of results, maybe try adding something else to your search to narrow the results. Most of the results are junk, just go through and find active sites. Once you find an active site, register using your new email.
Complete registration and sign in.
Find popular jobs, maybe some with the most "likes" etc. Copy down the user name of those jobs.
Click on the change password link on the right side of the page (usually) you should be at a url with change_pass.php or similar
This is where firebug or any live html editor comes in handy.
However you edit the code, find the line (it may look a little different site-to-site)
<input name="changepass" type="hidden" value="Update" />
<input name="auser" value=[YOUR USERNAME] type="text" />

Simply replace your username with the target username and fill out the password fields like you were changing your own password. Click on home and you should now be logged in as your target.