1. We are going to create this orchid thingy. So take a good look at it, because we need to store its form and colors into our brain.
Now start a new photoshop image and add a green background. You should always choose the dominant colour as your background colour. In this case the leaves will be green, so choose green as background. I heard this was so that mip-mapping wouldn't destroy the look of the model.
Now you should and paint the leaf shape, I've just selected a square to demonstrate how I paint a leaf texture (this is only my second time, so if you know of a better way, please email me, I always like to learn new stuff). Now paint some dark spots and light spots to make it a bit interesting.You can use the dodge&burn tool for that.
2. Now, I noticed that those leaves had very bright edges, so I tried to paint something like that.
3. Now create a new layer and choose black as your color. Paint some detail on your texture, and change the brush size now and then to make different elements. Duplicate your layer and hit CTRL-i to make it negative so that the black becomes white. Now move this white layer beneath your black one and move it 1 pixel to the right and 1 pixel to the bottom. Join these 2 layers together and then set the blend mode to overlay and play with the opacity until you get something that looks reasonable.