You'll absolutely need to cut back the siding and attach directly to the home. For the siding, you need two custom tools. The first is a small hook for unhooking a piece of siding from the piece below. Use this to remove all the pieces that are currently where you want to install the plate. Get a role of tyvek tape to cover up all the holes left from removing the existing siding. Then, snap a chalk line across the wall from screw to screw of the existing siding so that your reinstall will be perfectly straight.
And the second is a punch that is used to attach the siding to a special insert (undersill trim) that goes in the J-channel when you don't end at the exact top. You cut the siding to the height you need (measure twice) and then the punch creates small tabs that stick out from the siding (only 3 sides of the small hole are punched). The undersill trim goes inside the downward facing J-channel to receive this punched piece.
The top piece of J-channel (which is an inch or so long to line up with the side pieces of J-channel), you cut back an inch along creases to create a small tab that folds down into the J-channel on the sides. This helps create a water path that hopefully doesn't go behind the siding.
The next tip, when cutting, you can either use tin snips or a circular saw. We have a dedicated saw to siding on the job site and the blade is mounted in reverse. I believe this results in less tearing of the siding giving you a nice smooth edge.
One last tip, don't over-tighten the screws, the siding should be allowed to float right and left for expansion. The rule of thumb is that your fingernail should be able to get under the screw.
If this is confusing, go out and look at how siding is installed around a window.
Here are some of the tools that I referred to (I don't think you'll need the nail hole punch):
And a good description could be found at this site.