I never let in on my nail art secrets ;-) but i guess...I can..seeing as you're fellow artistes ;-)
Sponging is easy...paint on a pale base coat like white, pale pink, blue etc. take a darker colour...blob on the paint onto a palette or even a piece of paper!! Dab your sponge into the paint and dab it onto the tips of your nails lightly. Then sponge on the pale base again over the nailm not too heavily though. Then sponge on the darker colour. Then add top coat to blend it a bit.
Good colour combos for these are pale pink and bright pink, baby blue and dark blue, white and gold, white and bright pink, yellow and red.
Watermarbling is very messy!! Fill a cup or jar etc with cold water. Take a nail varnish and drip a drop of varnish into the water. It will spread out into the wwater. Add another colour into the water. get a cocktail stick and swirl the 2 varnishes around to make patterns. Dip your nail into the floating varnish and take it out to see the pattern on your nail. this does require practise! I usualy paint the nail white first, then watermarble them.
Good colour combos are white and pink, white and blue, white and red. basically..pales and darks, cos they contrast well. Glitter polishs tend to disintergrate in the water, so avoid them and shimmery polishes awell.
good luck
xxlaylaxx