Open ACID from the desktop icon or the "start" menu. Either choose the demo song installed with the program, one of your own compositions or create a new song. Make sure there is at least one track showing on the left of the screen.
Add some audio. Select the pencil tool or paint tool from the top menu bar and draw in some audio on any track so you can see the waveform displayed on screen. Draw in 10 bars or so.
Start editing. The pencil tool can be used to adjust the start and end points of the waveform or you can add extra sections further on in the track. The cursor will change to a double-headed arrow to indicate that it will adjust the length of the part. You can also place the cursor in the middle of a section and press the "s" key to split the part at that point. This enables you to delete sections in the middle of a longer part.
Adjust the volume and panning levels automation envelopes. Select a track and expand it so you can see the volume and pan controls, then choose "envelopes" from the "insert" menu. You can draw in lines that will affect the volume and panning of the track, allowing for fade-ins and panning effects. The same tools will work to edit MIDI parts and audio you have recorded into the program yourself.