We'll start with with iMovie if you plan on using Quicktime Pro click here.  

Step 1:  If you did not create your video in imovie you will need to import it first.  This is simple.  Click File/ Import, and choose your video.  

Step 2:  Once your movie is ready to convert click File/ Export 

Step 3:  Make sure the Quicktime icon is selected at the top of the pop up box.  Then for "compress movie for" select Expert Settings.  Then click Share.

Step 4:  This next box you will set the file name you want to save it as and where you want to save it.  The important part is the Use: box.  Set the Use: to Broadband - High.  This does a bunch of things but more noticably changes the viewing size of the finished movie.  Normal DV is 720 x 480 in size.  The settings Broadband - High changes the viewing size to 480 x 360.  

Options Button:  This will let you customize you exported movie even more.  If you don't understand this stuff it is best to leave it alone.  One item to look for is your Bitrate.  Here the Bitrate is 672 kbs/ sec which is pretty close to the maximum Bitrate that Epic Underground allows.  The Epic underground parameters for uploading a video are a Biterate of 700 kbs/sec and maximum file size of 200 mb.  Depending on the length of your movie whether it's HD or not you may have to tweak these settings a little bit if you find the converted product to be larger than 200 mb.  The goal is to be as close to 700 as possible. 

Step 5:  Click Save.  I saved a 2 minute video at these settings and it was 14 mb in size.  The original size was around 500 mb.