URL hacks are fairly easy. You just need to reference the ID from the page in the URL. Here's an example of the standard URL when creating a case with a specific record type:
500/e?retURL=%2F500%2Fo&RecordType=012500000009uny&ent=Case
e? = edit page
retURL = url to return user to (URL Encoded) use http://meyerweb.com/eric/tools/dencoder/ for help encoding a special URL
Recordtype = recordtype ID (not required, that's only if you have multiple record types)
ent = entity reference (here it is cases, if we were doing opportunities it would be ent=opportunity)
Now if you want to add custom fields to pre-populate when the user clicks a button then append '&' after case and add the ID of the field.
(using Chrome)
Now the ID of the field is found by right clicking and inspecting the field on the edit page of that object. For this one we are using the case object so we want to navigate to cases and click 'new'. Now you're in the new page you'll go to your field and right click you'll get something that looks like this: (for custom fields)
<input id="00N50000003N3EX" name="00N50000003N3EX" size="20" tabindex="8" type="text" value="">
copy that ID 00N50000003N3EX into the URL and =
after that you can set it to anything.
I set mine to {!Opportunity.amount} because I put my button on the opportunity page.
Now my URL for the 'New Case' button is: /500/e?retURL=%2F500%2Fo&RecordType=012500000009uny&ent=Case&00N50000003N3EX={!Opportunity.Amount}
which will populate that custom field on the case with the amount from the opportunity when the user clicks the button. ALSO make sure that button is set to 'Display in new window'
