Check out this thread where I vcarved two-color HDPE plastic sheets
You can use Advanced V-carving like Will said, but depending on the size of your letters and the look you are trying to achieve, a regular varve may do.
The thing with HDPE is that it likes (very) high feedrates, because plastic melts so easily.
Use low RPMs, bump the feedrate (it is counterintuitive but it will grant cleaner results). If you get strings of plastics rather than chips, you are not going fast enough