The "taping them to the window" trick does work, IF it's a window that gets at least a couple of hours of direct sunlight every day, so pick one that faces as nearly east or west as you can. And do check the sheet every week or so, because the process is highly variable.
One thing I learned the hard way - to avoid even the remote chance of condensation forming between the decal sheet and the window, and ruining the decals, I tape a strip of cardboard at the top and bottom of the sheet to act as a "spacer" between the glass and the decal sheet. Alternatively, if the decal sheet is curled anyway, you can tape it in such a way that the curled ends hold the rest of the sheet away from the glass.