Create a new card to show cistern water level

Hi,

Is there any tutorial how to create a new card? I have a few water leak sensors which I like to misuse as water level indicator of a cistern. Therefor I like to create a new card which shows a picture of a cistern and shows the water level based on the on/off state of the leak sensors.

My problem is that I have no idea how to start and I have no really found anything. If someone could point me to the right direction.

Thanks,
Sebastian