Open your preferred DAW, for example Logic or Pro Tools. The last edited session opens automatically. If this isn't the session you require, click "File," "Open Recent" and select from the drop-down menu. If you are adding a cappella vocals to an instrumental, open the instrumental track. If you are adding an instrumental to a vocal session, open the a cappella vocal session.
Click "File" and select "New Audio." This opens a blank audio track in the DAW interface, alongside the existing tracks. Click on the audio channel to highlight it. This assigns subsequent actions to that particular channel.
Click "File" and again but this time select "Import Audio." Browse for the file required, either the a cappella or the instrumental, depending on which way around you are working. Click on the file to load it.
Open the arrange window. Typically DAW have two windows, one for arranging audio and one for mixing it. The method for selecting a specific window varies according to which system you use, but it is typically a case of tabbing between the two. Your mixer window will have the channels laid out horizontally, with vertical channel strips. The arrangement window will have channels in a column, with horizontal sound wave graphics.
Click "View" and press "Zoom." This enlarges the sound wave graphics, which features peaks and dips to represent volume. Click "Tools" and select the "Hand" tool. This enables you to move audio left and right. Drag the imported audio left or right so that both tracks are aligned. The flat line at the beginning should end at the point. Use the time bar and beat grid map in the interface for reference.
Click on the a cappella vocal channel, this assigns future edits to the audio.
Click "Effects" and select "Reverb." This effect adds ambiance and creates a sense of space. Tweak the "Depth" parameter to adjust the intensity level of the reverb. This will help the a cappella vocal sit more easily in the overall mix.
Press "Play" so you can hear your mixes in real time.
Open the mixer window. Adjust the gain slider on the a cappella channel so the volume levels of the both the instrumental and the a cappella vocals are suitable. The a cappella vocal should be the most prominent feature in the mix, without being too dominant.
Review the mix after a one-hour break, make further tweaks to the mix if necessary.