Unless you use overheat (OV) to add damage, your mech will not need to track heat. If you use some of your OV value, you add that number to your table and buff your short and medium range damage accordingly. Of course, the following turn you deal with the consequences of that action. If you choose to use all 4 in one shot, your mech will automatically shut down at the end of that phase, it will be unable to do anything next turn and its heat scale will return to 0 for the following turn. Anything else from 1-3 will affect your gunnery and movement until your scale falls back to 0.
Here's how to get your unit back to 0 after using OV:
Beginning the end phase as a shutdown unit automatically drops your heat level to 0 (and you restart). Of course for that entire turn, you're a sitting duck with a -4 to hit modifier placed on you and all your movement modifiers are ignored on top of that.
Not making a weapon attack in the current turn also reduces your heat level to 0.
Entering water of 2 or more inches in depth will reduce your heat level by 1 , as long as you didn't use OV that turn.
Clear as mud?