To directly output individual extra fields in your K2 overrides, do something like this (e.g. in item.php) to get the extra field name:
echo $this->item->extraFields->EXTRAFIELD_ALIAS_HERE->name;
To get the extra field value you would simply do this:
echo $this->item->extraFields->EXTRAFIELD_ALIAS_HERE->value;
echo $this->item->extraFields->EXTRAFIELD_ALIAS_HERE->rawValue; // for date type only
Simply replace EXTRAFIELD_ALIAS_HERE with the actual alias of the extra field you wish to output.
In modules, use:
echo $item->extraFields->EXTRAFIELD_ALIAS_HERE->value;
echo $item->extraFields->EXTRAFIELD_ALIAS_HERE->rawValue; // for date type only