Ok you're going to be making brushes like this newbielink:http://www.deviantart.com/deviation/33825878/
[nonactive]Step 1) Make a new image 350.350 white
Step 2) Make 4 new transparent layers
Step 3) Change the modes so they look like this
Step 4) On layer normal Goto>Filters>Render>Clouds>Plasma
Use these settings
BTW change random seed
Step 5) Desaturate it
Step 6) On grain merge layer do step 4&6 again
Step 7) Then Goto>Filters>Artistic>GIMPressionist
Step 8) On the multiply layer Goto>Filters>Renders>Clouds Solid Noise
Use these settings
Click new seed a few times
Step 9) On the Divide layer do the same thing but with these settings
Click New Seed a few times
Step 10) Merge the layers down into the normal layer
Step 11) Select the paths tool and make a nice path the size and shape of your brush don't worry if it looks too sharp
Step 12) Click path to selection
Step 13) Goto>Select>Feather by 40-80
Step 14) Pree Ctrl+I and then Ctrl+K
Step 15) Flatten the image
Step 16) Goto>Image>AutoCrop Image
Step 17) Goto>Image>Mode>Greyscale
Step 18) Save with .gbr at the end
And now you should have a nice brush like this.
