How to use a value from a Google spreadsheet into a google form for dummies

Question

Dear Programmers,

I am turning to you for a small help. This can be considered a repost, but I think this question, if it will be answered will help a lot of people who are not familiar with JS.

So, the scenario is like this:

I want to calculate and track my travelled distances using Google Docs, for this I have created a small form which contains a few fields: Date, Miles at Start, Miles at End, Route description.

Every time, Miles at End, logically are = to the Miles at Start for the next trip.

This is why I need a script that will populate the field "Miles at Start" with the last value of "Miles at End".

To simplify the work, I have dedicated a cell in the spreadsheet which contains always the last value.

This issue is treated here: Is it possible to 'prefill' a google form using data from a google spreadsheet?

But I don't really understand how the code works:

/**
 * Use Form API to generate pre-filled form URLs
 */
function betterBuildUrls() {
  var ss = SpreadsheetApp.getActive(); // I need to put here the spreadsheet name? and in what format?
  var sheet = ss.getSheetByName("Sheet1"); // I should put here the sheet name? in betwwen ""?
  var data = ss.getDataRange().getValues();  // I don't have a range, I have a cell that contains the value, How i put it here?

  var formUrl = ss.getFormUrl();             // Use form attached to sheet - Should I add something here?

From this I part, I am totaly out :)

var form = FormApp.openByUrl(formUrl); 
  var items = form.getItems();

  // Skip headers, then build URLs for each row in Sheet1.
  for (var i = 1; i < data.length; i++ ) {
    // Create a form response object, and prefill it
    var formResponse = form.createResponse();

    // Prefill Name
    var formItem = items[0].asTextItem();
    var response = formItem.createResponse(data[i][1]);
    formResponse.withItemResponse(response);

    // Prefill Birthday
    formItem = items[1].asDateItem();
    response = formItem.createResponse(data[i][2]);
    formResponse.withItemResponse(response);

    // Get prefilled form URL
    var url = formResponse.toPrefilledUrl();
    Logger.log(url);  // You could do something more useful here.
  }
};

Please help me!

Thank you in advance!