Some tips to follow:
1. Always cover your surface with a good quality gesso primer in order to ensure that the colours you are going to use are not absorbed.
2. It is better to use two or more different acrylic colours instead of painting your surface with one colour. Apply the darker one with a brush and use the lighter colour(s) with a sponge. Or use the sponge for both colours. Ensure that the first layer is dry before adding the second colour. Pad the colour with soft movements. Your sponge must be wet but well drained. Do not put pressure on the sponge otherwise you will end up with circle patterns.
3. When your painted area is dry, brush an acrylic varnish with quick movements to protect your colour.
4. You may combine this technique with decoupage as the image shown on this post. The area on the right is painted with a sponge using two different blue tones and an off-white third layer.
5. Care: If you are going to use tissue paper, rice paper or other material for decoupage, never paint your surface with colour. Once you glue your paper on the coloured surface, the colour will come through the paper and ruin the image. The white gesso primer is all you need. Do not forget the final touch. Acrylic varnish to protect your creation.
Happy Creations!!