Ive used it once, and will this spring. It is very easy to do. First wash my car with a mixture of Turtle Wax soap and dish soap, to get the wax off. Then get a new bucket with Turtle Wax soap and wash the car in segements (fender, hood, door...so on.). After washing the car in that segment, I wet down that area, then work the bar in a circular motion, but make sure the area is wet. When finished with the whole car, wash it again. Yea, I know, I'm anal. Then dry it off with a soft towel. Now make sure a coat of wax is applied to the car asap, because all the wax is gone and the clearcoat is not protected. Personally, I like the Mothers 3 step program. It made my Dakota and Eldorado look sweet. Then to top it off, apply a coat of polish. I hope this helps.
As far as difficulties you might have encountered in using the claybar: Two things to remember. Keep it wet. Keep it warm.
I usually don't mess with the claybar in the summertime unless I'm using it to get bugs off that were baked on. It always seems to be wintertime when I'm doing the entire car. So, obviously the claybar gets cold and hard.
In my garage I have one of those "radiator" style plug-in space heaters. It has hot oil running through its coils. Anyway, I use two claybars, one is always sitting on top of the heater staying warm & soft. I wrap it in a washcloth that has been soaked in detailing spray so it doesn't dry out.
Mequiar's doesn't give you even HALF as much detailing spray as you're going to need to do an entire car. Be sure to use a liberal amount of the stuff. Anybody's detailing spray will work, so why don't you just go out and buy a gallon? I have a gallon of "Finish Fast" and a gallon of 3m that I use when I've run out of the Meguiar's stuff.
Just spray the detailing spray on the car and on the claybar. Press the claybar down onto the car's wet surface (or somewhere on the glass) to get it flattened out and VERY smooth. I try to get the Meguiar's bar flattened out to about a 2" by 3" rectangle. Then gently and lightly slide the claybar over the wet surface of the car. Use a back & forth motion, against the grain of any swirls or scratches you know about. Work about 1 sq. ft. at a time.
When I'm doing the FULL detailing job, I'll wash the car once with dish soap, then claybar. Wash again with a VERY MILD mixture of dish soap. Dry. Polish/buff. Glaze. Wax. Wash with a car "shampoo" that enhances the wax.
Wax again as often as you want to. You should be all set for a year, unless you get bugs baked onto your front bumper. The more wax you have on the car, the less likely that is to happen.
IF YOU DROP THE CLAYBAR, wash as much of the dirt off of it as you can. "Shave" or "cut" the rest out with a knife, then heat it up and massage the clay, folding it over numerous times until you see a clean surface again. If you leave even one grain of sand in your claybar, you're SCREWED. Throw it away and go buy another.
Clay bars can be a great way to revive old paint, but they aren't miracle bars. The key with them is to keep them lubricated just the right ammount. You want the bar to slide over the surface, but also have just a little bit of a grip (it's safer to be on the slick side if you are unsure). Be sure and fold the clay onto itself often to keep a clean side down. If you get any particles in the clay be sure and pick them out immediatly. They do a great job of pulling off all those small particles in the paint, but they won't remove swirls or oxidation or other more severe problems you might find on older cars.
It is recomended to clay 2 or fewer times a year, if you live in an area with lots of industrial fallout you might want to consider more than that, but usually it's not needed. What does work well is to 'spot clay' the areas that need it before you do a good polish, wax/sealent combo. It works great for removing tree sap and other compounds that seem to be 'stuck' to the surface.