Create a new document 6x6 pixels with transparent background. Take a pencil tool, fix it to 3 px and then click down right in the middle (while doing this don't touch the bottom and right egde, like in the picture on the left). You are left to define the new document like a pattern that you need to use later , so Edit> Define Pattern - name it like wish and click Ok.
Shape Tool
Create a new document of the size 150x130px with white background. The color of the first plan is black and background color is white (button D). Choose a shape tool and with it draw a clover (like on the picture) or retake .psd file that is made for you by clicking on download. Then rastarize layer (clicking with right mouse in the panel Layers> Rasterize Layers or from the main menu Layer> Rasterize> Shape) and in the end that layer needs to be connected with the white background (Ctrl+E or Layer>Flatten image).
Use Filter> Pixelate >Mosaic with the meaning of: 6. Then Image> Adjustaments> Threshold. Without any changes click Ok. Now with the Magic Wand Tool click on the black surface to get the selection. Create a new layer and fill the selection with the pattern by doing this: Edit>Fill from the drop down menu Use: use the pattern and than find a pattern that you've made in the beginning.
Marquee Tool
Remove the selection, take a tool for the selection (for example Rectangular Marquee Tool) and click in the open space and then click on Background in the panel Layers and click Ctrl+Backspace (this way the whole surface is overfilled with the color of the background , in this case: white). Now we can see the digital leaf of the clover. There is a new layer that can be combined like you wish. Like I did under.
The example is illustrated in the smaller size, but real and better effects you can see in the bigger sizes of the picture that you wish to make this way. If you want to use the effect on the bigger picture you can make it bigger by one square, just be careful while constructing. Lets say, if you create a pettern of 7 px, meaning of filter mosaic also needs to be 7 px, if you don't want the squares to be cut. Don't feel afraid from experimenting with different shapes and sizes of the tools. Feel free to try it!
Pencil Tool
Shape Tool
Marquee Tool 
