This may seem simple to others, but I searched around for this for days. If you want to sort a list by one of the Custom Fields, do something like this:
<txp:article_custom section="attorneys" limit="100" form="jumpmenu" sortby="custom_4" sortdir="asc" />

This needs to be in a <txp:article_custom /> tag.

The code above is used to sort all of the articles in the attorneys section of a website by their last name(Custom Field 4) instead of their first.

I.E. James C. Waddoups comes after Kelly J. Latimer.

My thanks to chrisrhee for pointing me in the right direction.

* 2011: The above information is outdated. <txp:article sort="custom_3 asc" /> now works.