When flying an ultra light and I have a tail wind, and, with the engine at idle how do I slow down to land ?
You don't change anything expect perhaps use short field landing technique to minimize your landing groundspeed, but otherwise you take the high ground speed. In light winds this isn't that big a deal if the runway is long enough.
I regularly land with minor tailwind components, under 5 kts, when it's more convenient and there are no traffic conflicts and space isn't critical. At a very large uncontrolled airport (10000 ft runway) I used to fly to regularly, where I had to share the runway with arriving large jets hauling freight from time to time, I would often use the reciprocal runway when following a cargo jet's arrival, taking the tailwind, in order to completely avoid the jet's wake, as long as the tailwind component was under about 12-15 knots. When you have all that room, the main issue is tire wear from the high touchdown speed.
Glider pilots train for downwind landings all the time, since it's a normal result of a low altitude rope break. The biggest problem isn't so much the high landing ground speed, it's avoiding overshooting your intended landing zone with the huge increase of effective glide angle with the wind behind you. Glider students are often very shocked when they discover this.
Even in an ultralight with a fairly steep glide angle, if you did a turnback to land downwind after an engine failure from 400 ft, you might also find your biggest problem is not overshooting, but in any case you would just use the minimum normal airspeed (short field) to land.
I think this anecdote bears directly on your question--
I often shoot downwind landings in winds up to 10 mph in a lightweight radio-controlled model airplane of tailwheel configuration. The key is to fly by reference to airspeed, not groundspeed. (One way to accomplish this in a radio-controlled model airplane, which lacks an airspeed indicator, is to learn where the elevator trim control needs to be set for a nice power-off glide, with hands off the controls, that is comfortably faster than stall speed and gives you a little extra airspeed "padding" to allow an authoritative round-out or "flare".) Avoid any temptation to deviate below this selected target airspeed when landing with a tailwind, just because the groundspeed is high.
Downwind landings often actually tend to be smoother than upwind landings, because if the aircraft accidentally "balloons" upward during the landing flare, the wind gradient will be working to reduce the airspeed, not to increase the airspeed, reducing the height gained during the "ballooning". Conversely, if the aircraft is dropping-- for example if the aircraft is rapidly settling toward the ground after excessive "ballooning"-- the wind gradient will be working to increase the airspeed, not to decrease the airspeed, so this tends to lead to a smoother touchdown.
In an aircraft of tailwheel configuration, however, it's often wise to limit your downwind landings to touch-and-goes (or touch-roll-and-goes, with an extended interval of high-speed taxi on the main wheels with the tail in the air) rather than attempting to slow to a full stop. As the aircraft decelerates with the wheels rolling on ground, a point comes where the airspeed over the control surfaces drops to zero, and then reverses, making the plane extremely vulnerable to ground-looping. Tricycle-gear aircraft are generally a much better choice than tailwheel aircraft, for full-stop landings with a tailwind!
A few more words of caution-- the overshooting tendency on final approach with a tailwind is not caused by the high groundspeed alone. The wind gradient actually reduces the aircraft's sink rate. While we've already noted that this can lead to a smoother touchdowns, it can also move that touchdown much further down the runway than the pilot intended, unless he or she adequately anticipates and allows for these effects. And you should also keep in mind that if you attempt a go-around with a tailwind, not only will your high groundspeed reduce your climb angle or gradient, but the wind gradient will actually reduce your climb rate as well, especially in the first few tens of feet or meters above the ground.
If you really want to slow the aircraft on final approach for the slowest touchdown that is safely possible, you do it exactly the same way when landing with a tailwind as you would do when landing with a headwind (e.g. when landing on a rough field) -- apply flaps, carry some power so you can safely operate a bit on the "backside of the power curve" without incurring a tremendous sink rate, etc-- to the extent that these techniques are appropriate and applicable in your particular aircraft. Whatever you do, don't let the high groundspeed influence you into choosing a slower-than-appropriate airspeed.
See also the answers to this highly-related ASE question: How to land a sports plane with tailwind? . Note several references to the effects caused by the wind gradient.