Here's the simplest setup:
- set Input 1 to MIC and Input 2 to something to which a physical device is not connected (e.g. Aux). Do not use your guitar and disconnect it form the HD500.
- plug your mic into the MIC input and adjust the Mic Trim setting until you get a strong but not overpowering incoming signal.
- create a single path preset and disable the amp (you can assign a guitar amp model if you wish but I suggest starting without one).
- add a compressor, one or more EQs, a little reverb and perhaps chorus or delay. Go easy - overprocessed vocals can sound really bad.
- tweak the above FX until you get a sound you like