Well for the basic texture this is what you do.
1. Get yourself a picture, the more colourful the better. If you're programme has an option like Gussian Blur (or something similar) set it to 50.00. You need to make sure there are no traces of the picure left, just the colours.
2. Once you've done that, if you find it lacks colour, find another picture and do the same thing, then place it on top of picture number 1, and overlay it (or use the soft option).
3. Alternatively, if you don't want to use picture upon picture to get a good colour, select a colour from your tool pallette (hopefully something different to what is already on the blurred picture), floodfill over the image, and set that to overlay or soft instead. You can keep building up the colours until you get something you like.
4. Then take an interesting brush and cover the coloured image. (White or black for this works best, rather than other colours). Overlay or make it soft depending on how you think it looks. Then choose another brush and cover your image as well. Again choose which setting looks the best. (For a neat look, use a brush in white and then use the same brush on the same layer in black, it gives a patchy colour effect)
5. Keep adding brushes until you like the effect you have.
