You can design a piece of a tree by following some simple steps that are given in this example to make it easily.
Take a New File: We have to take a new file to modify, the file size depends on you choice.
Set Color: Now set the foreground and background color as given below...
Reticulation: Go to Filter Menu > Sketch > Reticulation and make setting.
Plaster: Go to Filter Menu > Sketch > Plaster and use these setting.
Levels: Go to Image Menu > Adjustment > Levels and use setting.
Recommend the tutorial