How do I access the custom field value for 'each' issue in project?

[Alex Chan]

I want to eventually SUM a couple custom field values for a given Issue / Ticket. 

In my first iteration, I am just trying to retrieve a single custom field value.

QUESTION: How do I access issues? I want to "getCustomFieldValue(customFieldName_userPain)" for a given issue, but I'm not sure how I'm supposed to be iterating over issues.

import com.atlassian.jira.component.ComponentAccessor
def customFieldName_userPain = "userPain" 
def customFieldManager = ComponentAccessor.getCustomFieldManager()

def customField_userPain = customFieldManager.getCustomFieldObjectsByName(customFieldName_userPain)[0]
def score_userPain = [current_ISSUE].getCustomFieldValue(customFieldName_userPain)

return score_userPain

Note: if I revise line 6 to:

def score_userPain = getCustomFieldValue(customFieldName_userPain)

I receive Null for score_userPain. I believe I'm getting null because I'm not referencing any issue in particular ... (I need a high level explanation for all of this haha) :)