Essentially, you need to draw a junk sail with at least the same sail area as the working rig of the original sailplan, preferably about 10% more. The centre of effort of the junk sail needs to be in the same place as the original, though some say a bit further forward. This means your new mast will have to shift forward as well. It will be keel stepped, and well braced where it comes through the deck.
Basically it is simply a lugsail. It has a halyard that attaches to the centre of the yard. The halyard is slightly offset at the masthead, on the side the sail sits. The multiple part sheet attaches to the aft end of the boom and battens. There are a few other control lines, such as a parrel that holds the yard to the mast and another than tensions the luff. Details of these are illustrated in the literature.
However, it is impossible in a single post to give you anything other than a brief outline like this. You could get a professional to design the rig, and even build it, though the cost will be hard to justify. I think David Tyler's suggestion of initially reading Arne's pages is a good place to start, plus Practical Junk Rig of course. If you are an experienced sailor, you should be able to translate your knowledge easily enough, paying attention to the different ways of sheeting the sail, rigging the halyard, etc. If there are JRA members near you (look up the members list), an examination of their boats would be invaluable.
Good luck!