I want to know best way to clean the humidifier and how often?

It all depends on a couple of factors.

First is the type of humidifier your using. Portable or whole house connected to the furnace. Steam, ultrasonic or filter. Your unit's instructions is the best guide as to how often you need to clean it and this can vary from every day to once a heating season.

My recommendation is to watch and clean the unit as soon as you see scale forming. Then in the future, reduce this to just before you see scale forming.

The second is the type of water you are using. Hard tap water is the worst and usually will require every day for small portable steam or ultrasonic units to once a week for filter models. For central whole house humidifiers, once a month (perhaps as little as every two weeks if your water is really hard, >10 grains hardness). There is no fixed formula for this, the idea is to perform on-going, preventive maintenance.

Soft or conditioned water will contain the same amount of minerals as hard, but the softener has replaced the calcium minerals with sodium minerals. (Sodium bicarbonate actually).The result is the humidifier will still form scale but it much easier to remove. With soft water scale, you can just break up the scale with your fingers and the filters can just be submerged in clean water and the salt scale will re-dissolve.

Third, with distilled water, you may only need to clean any of these units annually. (No minerals to for scale).

With any filter humidifier, consider using a commercial bactericide to prevent the growth of bacteria when the water sits for long periods (>14 days). Not so critical with soft water as the salt will retard the growth of bacteria, but very important with hard or distilled water. Don't use this product in ultrasonic or steam humidifiers as it will be carried into the air along with the water vapor, and can be considered as toxic.

With hard water, use a 50/50 solution of white vinegar and water to dissolve the scale buildup. This works best if done on a regular basis. If left go too long, it become impossible to clean properly. You need only use regular water when using soft or distilled water in your humidifier.