It is something you can DIY. You learn the skill, just like any other. It is mastery of cutting with a blade(razor or olfa type) and ability/patience in cleaning the glass. Having a perfectly clean garage to work in also is a must. Not to mention applying it cleanly.
A 911 has many cuts (glass) that require an expert's touch or at least someone who has done it before, plus as stated if its a targa only an expert can make the neccesary cuts (so that the film conforms to the shape of the glass) or heat shrink it (done on the outside of the glass and then applied inside.)
At least I would say you should practice on a friends car first
Kidding!
Regards,