I positioned the vent in an area of the windshield that I thought would work best for me. (See attached picture of the windshield with the vent installed) You could position it lower, but for me I found optimal air flow at the position I chose.
Here is a link that includes the instructions that come with the vent as well as pictures showing the steps required to mount the vent.
http://goldwingdocs.com/forum/viewtopic.php?f=11&t=6578
I would truly recommend removing the windshield from your Spyder. It takes only a few minutes and makes the marking and cutting of the windshield much easier.
I masked the cut area on the windshield with several layers of blue painters masking tape, marked the cut out, drilled two holes with a 7/16 inch drill bit and used a jig saw with a scroll type blade to completely cut out the vent hole. After I made my slightly undersized initial cut of the vent hole I used a rotary type tool (a hand file would suffice) to gradually shave the edges of the opening to ensure a snug fit of the vent. The marking cutting and fitting took about 3/4 of an hour and turned out to be easier than I first imagined. Overall I would rate this mod as being moderately easy once you get over the fact that there was no turning back once you started drilling and cutting. Felt somewhat like cutting the seat for the Utopia Backrest install, for those of you who have been down that road.
I took the Spyder out for its maiden "spring" run a few weeks back when the temperature was hovering at around 34F. I noticed a definite cool draft when I opened the newly installed vent while traveling at 70 miles per hour down the road!