— First you have to load the ABR file in Photoshop. Go to Edit > Preset Manager and Load the ABR file.
— Pick the Brush Tool and choose a color for the brush. I usually use white color for the space brushes. Add an Inner Glow and Outer Glow style to create colorful effects.
— The size of the galaxy brushes is around 1500px. If you use a smaller size, the brush becomes a little blurry. So, after you add the brush in a new layer, ?? I suggest you use the Filter > Sharpen > Smart Sharpen filter to fix this issue.