I need a manual for simply bathroom body analysing scale

These measurements are based on BIA (bioelectrical impedance analysis). As you stand on the scale, you stand on electrodes, which generate small electric impulses which pass throughout your body (and different tissues of course). Different tissues have different electrical conductivity and resistivity due to their different hydration (e.g fat tissue contains very small % of water, that's why its resistance is high), so the scale measures the % of different tissues in your body by analyzing the resistance(this analysis is quite complicated and not very accurate).

Probably there's a build-up of skin oil from your feet that is preventing it from reading properly. Clean off the top of the scale with a cloth dampened in soapy water, then rinse wipe it with a cloth dampened in plain water, and dry it. Be careful not to dribble liquid into the scale.

Note: fat measurement by this method is difficult to do accurately and consistently, For best results, make sure you're properly hydrated at least ten minutes before measuring, and wash and dry the soles of your feet first.

Yes. These systems work by what could be broken down as, "Bio-electrical Impedance Monitoring".
Meaning, they measure the amount of resistance in your body to determine the "volume %" of non-resistive materials (fat and water). Since bones and muscle are very dense in comparison, the scale can calculate the total weight of a person, then determine their "expected BF%). Fat is about 80-90% water depending on your genetic makeup, and it would only make sense that if you where full of fluids (either by ingesting large amounts or by fluid build up under the skin) that your BF% would read higher. My professional suggestion? Use it only after waking and before any morning drink or food.
(REMEMBER...THESE THINGS WILL NEVER BE EXACT!!!) Hydrostatic weighing in the only way to be 99%+ on your BFC (body fat composition)
GOOD LUCK AND PLEASE RATE THIS POST!!
P.S. one way to test this method is to hold something heavy in your hands and watch it consider that weight Lean Muscle Tissue. Then stick your hands in a sink full of still water (moving water posses a small electrical charge that could give a false reading) and watch your BF% increase.

http://www.niams.nih.gov/Health_Info/Bone/Bone_Health/bone_mass_measure.asp
this sight gives you the measurements

The BIA is bioelectric impedance analysis or your body fat. BIA is the method used to determine your body fat, it sends a small electric pulse through your body and using formulas that account for the time it takes for the pulse to travel through water, muscle and fat. Remeber that this measurement can be affected by different variables such as water if you have taken a shower or drank a glass of water before being measured so it is not always acurate. To obtain a more accurate measurement I reccomend using it first thing in the morning before showering or eating/drinking when there is less chance of variables. Please let me know if you need further help :) Have a great day!

Unfortunately, it's the scale, but it's not broken. It simply is telling you inaccurate information.
Some scales utilize Bioelectrical Impedance Analysis (BIA) technology. BIA uses very low levels of electrical impulses running from one metal contact point/lead to another. Your body acts as the connector between the two contact points, thus completing the circuit. Not all scales use this means for calculating body fat percentage due to poor reliability and high fabrication costs.This method of measurement requires very large contact points, as well as clean and moist skin exposure. If you have dry/calluced feet, which is normal for most Americans, the sensors can not accurately measure due to skin resistance. Dry skin acts as a resistor to electrical impulse. Electricity passes through muscle and fat at different rates. However, its measurements may be thrown off by how hydrated you are and how much you've eaten; it may be too inaccurate for anything other than comparative use. Users are encouraged to establish a baseline measurement and repeat measurements under consistent conditions over time to detect changes.
Many less expensive models simply use a body-fat calculator to average your imputed height in inches, waist measurement in inches and the current weight measurement recorded in lbs. The scale compares your measurement with the ideal norms inputed during the manufactoring process, thus giving you a comparison vs.imputed norms. People who are very, very fit can get numbers 3 to 5 percent higher than their true percent body fat. Because they don't have a lot of fat inside their muscles(visceral fat), the simple tape measure calculation test cannot adjust as it should indicate.
Conversely, if a person is skinny but not fit, this body fat test may yield a number 3 to 5 percent lower than his or her true percent body fat. Though they look thin, unfit skinny people really have more than the usual amount of fat inside their muscles, which you can't see from the outside.
In either case, the scale is not necessary a measurement tool, due to high variables of test inaccuracy and interuser reliabilty; but, it is a good tool for comparing whether or not you are gaining/losing body fat.
If you would like a more accurate tool for simply measuring your body fat, you can pick up some pinch-calipers online for about the price of a scale. They come with an instructional pamphlet, very easy to use.