Help please

I am trying to set up a very simple mailing list for a
few friends. What I know so far is this:

  Any email where the X-Rcpt-To field is mlist(_at_)herp(_dot_)com

    1: First verify that the sender's email address is found
       in mlist.ppl (a text file with one email address per
       line). If they are not listed, forward the email to
       problems(_at_)herp(_dot_)com and stop processing.
    
    2: Make the "Reply-To" field say "Reply-To: mlist(_at_)herp(_dot_)com"
    
    3: Forward the modified message to all email addresses
       listed in mlist.ppl

I've got a solution for Step 2:
  formail -i "Reply-To: mlist(_at_)herp(_dot_)com"

And Step 3:
  :0
  * ^X-Rcpt-To(_dot_)*mlist(_at_)herp(_dot_)com
  ! `cat mlist.ppl`

But I have no idea about Step 1, or the most important part,
tying it all together into one recipe for my .procmailrc file.

Can anyone help?

Bryan Mohr