Launch Ableton. Click the "Live Device Browser" icon on the left-hand side of the program window.
Click the arrow next to "Instruments" to expand the section. Click "Sampler." Drag and drop the Sampler onto the track labeled "2 MIDI."
Click the folder icon labeled "1" on the left-hand side of the Ableton window. Navigate to the sample you want to use for the bass line. Drag and drop the sample into the area of the Sampler window labeled "Drop Sample Here."
Click and drag the arrows at the top of the sample waveform to select the part of the sample you want to use for the bass line. Adjust the sample's volume, if desired, by clicking and dragging the "Vol." control.
Click the "Pitch/Osc" tab above the sample waveform. Click the "P. Env" button. Click the box underneath "Transp" and enter the number of steps by which you want to transpose the sample. To lower the sample's pitch by an octave, for example, type "-12." To lower it by two octaves, type "-24," and so on.
Double-click the first clip slot in the "2 MIDI" track to create a new MIDI clip. The piano roll editor opens.
Double-click notes in the piano roll editor to add them to the bass line. Right-click the piano roll and select a value in the "Fixed Grid" section to change the lengths of the notes, if desired.
Click the "Play" button on the MIDI clip to listen to the bass line.