How you make skins is simple and yet oh so complex.
You make a bitmap. Usually you make a layered file and export it as a bitmap. You have to "draw" it so that the proper parts are in the right place on the bitmap, and the end results looks like a plane when the skin is "wrapped" around the 3D model.
However, HTC only accepts historically accurate skins, so taking Hartmann's tulip pattern and putting it on a 110G-2 is not historical, same as painting bright red and yellow flames on a P51D. Neither would be accepted unless you could prove it existed historically.
However, if you would like to practice or start learning, or just want to play with the skin offline, you could easily put tulip petals on the default 110G skin. I simply wouldn't submit it to HTC.