You'll have to look at the field names. The list of names for standard fields is for example here: http://phollaio.tumblr.com/post/22058016777/how-to-obtain-a-field-id, for custom fields it's usually the field Id when viewed in setup (in case of doubt use Firebug or similar tool to inspect the HTML).
Something like this should give you the general idea how to start:
It's up to you if you want to write full URL yourself or do you prefer to read a bit more about
URLFOR function (quite powerful). Generally speaking this will populate Account nicely if you've fired it from rel. list under Account - from other places it is likely to return empty string so you might have to use
CASE() or similar to make it more generic (depends on when do you plan to use the button).