sponging is a Glazing techinque. you need a base colour (the colour you want to show through) in a satin finnish. then you need the glazing material or top coat. this can be a glazing compound that has been tinted or paint that has been watered down a little bit so its thinner and can flow better.
you will also need :
a sea sponge (which can be found at most craft stores, or paint supply stores)
gloves,
a paint tray,
a bucket of clean water,
plastic dropcloth to protect what you dont want paint on.
painters tape.
prepare your work area.
1: pour some of your glazing compound into your paint tray.
2: dampen your sponge with water and ring out till almost dry. dip sponge into glaze wipe off excess.
3: tap sponge on wall varying the pattern leaving puzzle piece (not straight) edges.
redip your sponge and repeat till your wall is complete. see video...
TIP : you should ring out your sponge completely in water occasionaly to prevent getting too much glaze buildup