Single-tap to the filters icon on the left toolbar will open up a beautiful range of filters on the right side of your screen on the Photo Desk. Slide through the filter library and tap on any one filter to select it.  
To apply the selected filter, simply draw over the area of the photo you want the filter to be activated.  When tapped on the filter icon again, you will see an undo icon, redo icon, a fill bucket icon, and a slider. By tapping on the Fill Bucket icon, you can apply the filter effect you selected to the entire photo all at once. The slider will adjust the stroke size behavior of the filter when applying it manually by drawing over a photo.