Markers work a bit like watercolors.  It's best to start light and build up to your colors since it can be tough to lighten a color once you've gone too dark.  You can use a colorless blender to blend and soften the edges of your work.  Depending on the surface, markers tend to dry quickly so don't plan on making color changes.  Markers are appreciated for their immediate and colorful results, but the artwork is not intended to withstand the rigors of time.