what happened to my potatoes :(

[Two_Sheds]

how often should i think about watering them if i am growing them in a bag then?

Less often than you are

Watering is a skill that comes with practise. Most beginners think the soil has to be moist at all times: it doesn't.
I get my children at school to lift a dry pot and a watered pot, and compare how they feel: heavy, or light for its size? (the watered one is heavier of course).

If a pot is too heavy to lift up, then poke a finger down into the soil: is it really dry, or damp? Really dry needs a watering, damp doesn't. Hot sunny weather means daily watering, but it's not necessary in cool cloudy weather.

Seedlings are easier to guage ~ stand them in a saucer of water for a couple of mins, like this