How do I go about starting and getting a default divorce?

You will need to have your papers prepared and file them with the court.  After filing, a Citation will be issued and your spouse served.  In the case of an "absent spouse" (ie no known whereabouts), you can "serve" a divorce complaint via "notice by publication".  This is used when a respondent-spouse cannot be located by the petitioner-spouse.  Basically, the respondent is notified of the divorce proceedings by publishing such notice in a local paper in the area of their last known whereabouts. 

In either case, if after 20 days your spouse does not answer (ie they default) then you will file your final paperwork and appear before the judge on or after the 61st day.  A divorce will then be granted on the terms that you requested.