Using a MQ-3 Sensor .
A slightly more complex sensor.. but its really just the same.
Task 1. Surf the internet to find out what to do with this thing!!
The Arduino community is huge, and there are loads of people who have already done what you want
Data Sheets don't make sense.
Try to visualize it your self.
Once you have it Printing.. Its time to make something.
There are no rules and it is recommend to use as many components in your kit as possible