first of all, you need to understand how these sensors work.

1. the sensor itself is just a resistor (like a normal resistor, with 2 legs) which is constructed from materials that are affected in presence of a specific gas. the gas changes its resistance. but as it's obvious, these type of materials are not calibrated.
2. in order to make that resistor sensitive enough, we have to warm it. so the manufacturer puts a simple coil element near the resistor in the capsule and you connect it to 5v. it warms up (it draws a lot of current) and thus, the effect of gas on the resistor becomes greater and we can measure it easily. when they say connect and let it be for 24 hours they mean warming up the resistor. I think it's a little high. I used them after a few minutes of warming up but it depends on situation and desired precision.
3. the resistor and element, have separate pins and are completely independent. you have to connect the coil to 5v (because manufacturer made it like that) and use the resistor anyway you like. even with a ohmmeter.
4. as we have to use Arduino, so we have to measure a resistor with the ADC of Arduino. the procedure is straightforward. you put a resistor in series with a sensor and make a voltage divider (search that if you don't know about it. it's a fundamental thing to know in electronics).I think there should be a schematic in datasheet for it. and you measure the voltage with ADC.

and some notes :

• you have to use A0 pin of the module if you want reading the value like the procedure above. this pin is the output of voltage divider(the sensor + blue pot on the board) and should be connected to ADC.
• the D0 is just the output of another circuit present on board for another reason. it a comparator output. it's used for situations where you only need to do something (like turning emergency lights on and off) when the gas goes up from a level. you don't read anything with ADC. you just turn the pot to set a threshold (that's empirical) and digital pin get high and low according to that.
• the voltage divider's GND and VCC are connected to GND and VCC of heating element accordingly. so you have just one pair of GND/VCC

first of all, you need to understand how these sensors work.

1. the sensor itself is just a resistor (like a normal resistor, with 2 legs) which is constructed from materials that are affected in presence of a specific gas. the gas changes its resistance. but as it's obvious, these type of materials are not calibrated.
2. in order to make that resistor sensitive enough, we have to warm it. so the manufacturer puts a simple coil element near the resistor in the capsule and you connect it to 5v. it warms up (it draws a lot of current) and thus, the effect of gas on the resistor becomes greater and we can measure it easily. when they say connect and let it be for 24 hours they mean warming up the resistor. I think it's a little high. I used them after a few minutes of warming up but it depends on situation and desired precision.
3. the resistor and element, have separate pins and are completely independent. you have to connect the coil to 5v (because manufacturer made it like that) and use the resistor anyway you like. even with a ohmmeter.
4. as we have to use Arduino, so we have to measure a resistor with the ADC of Arduino. the procedure is straightforward. you put a normal resistor in series with the sensor resistor and make a voltage divider (search that if you don't know about it. it's a fundamental thing to know in electronics).I think there should be a schematic in datasheet for it. and you measure the voltage with ADC.

and some notes :

• you have to use A0 pin of the module if you want reading the value like the procedure above. this pin is the output of voltage divider(the sensor + blue pot on the board) and should be connected to ADC.
• the D0 is just the output of another circuit present on board for another purpose. it's a comparator output. it's used for situations where you only need to do something (like turning emergency lights on and off) when the gas goes up from a level. you don't read anything with ADC. you just turn the pot to set a threshold (that's empirical) and digital pin get high and low according to that.
• the voltage divider's GND and VCC are connected to GND and VCC of heating element accordingly. so you have just one pair of GND/VCC