Here's a video showing my step by step process of swapping out the boring gray WRX carpet with the blue STi carpet. This tutorial should work for all model year Subarus since the interiors don't differ drastically. But this will mainly benefit anyone looking to do the swap on the 2002 through 2007 models.
The actual install starts at 2:15 in the video (Just skip forward unless you want to hear talking)
STi Blue Carpet UNBOXING & INSTALL | 2004 WRX - YouTube
Hope this helped! I know a lot of people wanting to do it!