Make Some Cool Sponges

For practical purposes the use of the word sponge on this site will also include various foams such as those found in the cushions of furniture (couches etc.) the foam that is sometimes used in packaging, the egg crate type as used on beds, essentially foam is light and quite airy, i.e. foam padding.
There are denser foams that will work such as those already made into designs, preformed ivy leaves for example and fleur delis, available in the craft market, these however will hold little paint and you will have to reload often.
Foam carpet pad works well. Anything that will hold its shape and an amount of paint that can be pressed to your design... Sponges are easy to come by as they are a regular kitchen/cleaning item, and they come in many densities. Some even have scrubbers backing them, these scrubber backings have their own applications, luffa-like they would lend themselves well to highlighting tree barks among other things. I would think they might make great autumn leaves, leaving their impression after the fleshy part had gone. While their many applications and effects might elude me I elaborate because you might want to use them, for something.
Thick sponges will keep the paint further from your fingers and work the best for children and myself. Though difficult to cut with scissors they are easy to saw into shape with a large serrated knife, (bread knife) you can fine tune them afterwards with a pair of scissors. Some sponges are thick enough that you can put a different pattern on each of its faces.
Generally you will want to trace your pattern directly onto the sponge. If you are working with natural leaves for instance lay them directly on and trace with a felt tip marker or pen (some sponges/foams work best with one or the other) if your pattern is on paper it is the same. Lay it on and trace.
If your pattern has intricacies that you wish to incorporate onto your sponge you may want to cut it up after you have done the general outline and trace on its different pieces. But if your pattern is a forgiving one such as leaves don't go to all that bother. Simply approximate the major lines, eyeball it and draw them freehand! If you draw something onto your sponge you don't like, scribble it out (so you don't forget and accidentally cut it wrong later) and go on.
To cut out the veins or lines you have drawn fold the sponge nearly in half and snip with scissors or saw with serrated knife while the part you are removing is thrust up and out on the foremost curvature of the sponge. For natural patterns it is not necessary to have these lines trimmed out as neatly as you might think so don't spend a lot of time. After you try it with paint you'll know if you want to make any adjustments.

The veins you remove will look equally nice as those you have painted and its far easier to delete them from your pattern and have them show up in relief than to paint them on.
You can make patterns from rubbings by laying a paper over a leaf (some leaves will work far better than others) and scribbling or rubbing across it in one direction with a pencil or a pen (pencils are generally preferred). Save these until you will have time or the proper sponges if the leaves you want to work with will not be available then. * Colored pencils make beautiful rubbings for greeting cards etc. * Children can do a very decent job of this for you so if any are handy select your leaves and delegate.
FACT... I have a sponge grape leaf I traced that's over two feet across which I use as a part of set. I have used it to paint on windows and walls both with temperas and permanent paints. The life of a foam is a long one!
'If you hear a voice within you say you cannot paint, then by all means paint and that voice will be silenced.' Van Gough