I've used Jib Downhauls on several sailboats and found them to be simple and very effective.
The simplest design, and the one you should try FIRST, consists of a light line attached to the head of the jib at the halyard shackle or the jib head grommet which then goes through a turning block at the stemhead on its way to the cockpit.
When of appropriate length and loose, the line proceeds up the forestay when the jib is hoisted and is then snugged down a bit so as not to fly all over the place and is then cleated off. When ready to douse the jib, simply uncleat the downhaul, release the jib halyard, and haul in on the downhaul to bring the jib quickly down to the deck. When all the way down cleat off again to keep the jib locked down to the foredeck.
There are other variations on the same theme which you may want to try later as you become more familiar with downhaul designs.
Hope this helps.