Like photoshops bucket tool pattern or just want a pattern brush. Use this method to get that effect. I'm going to use a screentone texture that is inverted.
Go to your paint tool sai Folder
Open Brushtex folder
Open a bmp image in photoshop or sai
decorate it in greyscale. You can see my texture above, it's was done in photoshop as render difference clouds. Rename the file to something you will remember. I called mine Lightening because when it's used it looks a bit like lightening or coffee stains.open the CONF file called Brushtex. Open it in Notepad. It will look something like this>>
write this: 1,brushtex\[the name of your bmp file].bmp Make sure on the end up the image file name. Exactly like in the image above. Mine was lightening so I wrong it: 1,brushtex\lightening.bmp and after you write this, close and save,