Open Logic Pro. Since Logic is available only for Mac OS X, you can find it inside your "Applications" folder (Macintosh HD > Applications). Click "File" > "Open" to select the Logic project that you want to edit.
Click the track containing the notes you want to quantize. Your tracks look like long horizontal strips and appear on your main project window, called the Arrange window. In the left column, you can find the track titles, like "Audio 1" and "Inst 1." Each track contains a layer of your music recordings, such as a guitar line, a synthesizer or a drum beat. To select a track for editing, just click the title in the left column.
Drag the horizontal scroll bar (beneath the track list) to the right until you find the region containing the notes that you want to quantize. A region is any individual recording appearing on the track, like a guitar riff or an eight-note bass line. If your desired region appears at the beginning of the song, you do not need to drag the scroll bar. Otherwise, drag it until you find a desired region.
Click the "Quantize" menu in the upper left corner of your project window, on the "Inspector." When you click the menu, you will see a list of quantizing options with different numbers. Choose a number based on how precise you want your timing. For example, if you want to line up your notes so that one note plays on each beat, choose "1/4-note." To allow two notes per beat, choose "1/8-note." For four notes, choose "1/16-note."