I am going to assume you already have your draws for the part-worth you want to compute the HPD for and that your draws have a unimodal (one peak) distribution.
If you are trying to compute a 95% HPD, then you are looking for the *narrowest* interval that contains 95% of your draws.
Here are the steps in Excel:
1) Sort your draws for the column you are trying to compute the HPD for.
2) Figure out how many draws represents 95% of your data (assuming you're looking for a 95% interval), which would be:
k = (size of sample)*(0.95)
3) Compute 'item k' minus 'item 1'
Compute 'item k+1' minus 'item 2'
repeat until 'item n' minus 'item k-n'
4) Whichever result from step 3 is the smallest, those are the bounds of your 95% HPD.