How Do You Float Mount a Watercolor?
Start out by cutting two pieces of mat board to the same size as your frame. The first piece will become your window mat; the second piece will be the background against which you will mount your watercolor. Now this background mat may in turn be backed by foamboard to create stability, prevent bowing and pack the frame, but the actual mounting will be done against the mat and not the foam board.
On the back of the mat mark out in pencil the area that will be covered with the watercolor when it is mounted against the front of this same mat. You are delineating a safe area where you can cut slits and know that they will be concealed by the watercolor when it's mounted. Within this safe area, about three-quarters of inch below where the top edge of the watercolor will be positioned, cut two narrow slits just wide enough to feed tape through.
You should use linen mounting tape for mounting watercolors because its tensile strength is adequate to hold the weight. Feed tape through the slits so the adhesive side is out as it hangs down the front of the mat. At the back of the mat, adhere the tape in the area above each slit. Since the tape is now adhesive side out against the face of the mat, it's a simple matter of pressing the watercolor against the tape and "Voila!" you have it.
By cutting slits in the background mat you introduce the tape at a point far enough down from the top edge that it can't be seen above the deckled edges but still high enough to provide a good point from which to suspend the paper.
Now all you have to do is cut your window mat so that it's larger than the watercolor, place it over the watercolor and you have it, a float mounted watercolor.