The soap and water thing is normally done with decals. If you are useing MONOKOTE, then spay Windex on the area first (The Windex ONLY WORKS WITH MONOKOTE ). The solutions activate the adhesive on the decal (covering). If you are using something other than Monokote for your trim/decals you will need to iron them on.

On my planes I have 2 stripes, each about about 2" wide, on the bottom of one wing only. I normally have the bottom of my wings a dark color and the stripes white. The contrast is needed in order to see the stripes at a distance. By having the stripes on one wing, I can determine which way the plane is going and whether it is right side up or upside down.