Create

//  Create a Map to store all the data of an Estimate
Map<String, Object> nsEstimate = new Map<String, Object>();
    nsEstimate.put('classification', new Map<String, Object>{'internalId'=>'5'});
    nsEstimate.put('currencyRecord', new Map<String, Object>{'internalId'=>'2'});
    nsEstimate.put('customForm', new Map<String, Object>{'internalId'=>'124'});
    nsEstimate.put('department', new Map<String, Object>{'internalId'=>'2'});
    nsEstimate.put('discountItem', new Map<String, Object>{'internalId'=>'-6'});
    nsEstimate.put('discountRate', '-15');
    nsEstimate.put('dueDate', DateTime.newInstance(2020, 12, 31).getTime()/1000);
    nsEstimate.put('entity', new Map<String, Object>{'internalId'=>'362'});
    nsEstimate.put('entityStatus', new Map<String, Object>{'internalId'=>'8'});
    nsEstimate.put('expectedCloseDate', DateTime.newInstance(2020, 12, 31).getTime()/1000);
    nsEstimate.put('externalId', 'SFId: a012w00000Oyssh');
    nsEstimate.put('memo', 'Order 25');
    nsEstimate.put('otherRefNum', '4343399');
    nsEstimate.put('probability', 35);
    nsEstimate.put('title', 'SF Estimate');
    nsEstimate.put('tranDate', DateTime.newInstance(2020, 09, 07).getTime()/1000);
//  Initializing the Estimate Custom Fields
    List<Object> estCustomFieldList = new List<Object>();
    Map<String, Object> estCustomField = new Map<String, Object>();
        estCustomField.put('fieldType', 'select');
        estCustomField.put('scriptId', 'custbody_customlist');
        estCustomField.put('valueLookup', new Map<String, Object>{'internalId'=>'2'});
        estCustomFieldList.add(estCustomField);

//  Adding the Estimate Custom Fields
    nsEstimate.put('customFieldList', new Map<String, Object>{'customField'=>estCustomFieldList});

    //  Initializing the Estimate Line Items
    List<Object> lineItemList = new List<Object>();
    Map<String, Object> lineItem = new Map<String, Object>();
        lineItem.put('description', 'A weapon for shooting arrows');
        lineItem.put('item', new Map<String, Object>{'internalId'=>'239'});
        lineItem.put('quantity', 1);
        lineItem.put('rate', '999');

        //  Initializing the Line Item custom fields
        List<Object> liCustomFieldList = new List<Object>();
        Map<String, Object> liCustomField = new Map<String, Object>();
            liCustomField.put('fieldType', 'boolean');
            liCustomField.put('scriptId', 'custcol_checkbox');
            liCustomField.put('value', 'true');
            liCustomFieldList.add(liCustomField);

//  Adding the Estimate Line Item Custom Fields
        lineItem.put('customFieldList', new Map<String, Object>{'customField'=>liCustomFieldList});
    lineItemList.add(lineItem);

//  Adding the Line Items to Estimate
    Map<String, Object> estimateLineitemList = new Map<String, Object>();
        estimateLineitemList.put('item', lineItemList);
    nsEstimate.put('itemList', estimateLineitemList);

//  Initializing the RequestJSON Data to be passed.
Map<String, Object> requestJSONMap = new Map<String, Object>();
    requestJSONMap.put('estimates', new List<Object>{nsEstimate});
    String reqJSON = JSON.serialize(requestJSONMap);

Map<String, Object> reqObj = new Map<String, Object>();
    reqObj.put('version', '1.0');
    reqObj.put('action', 'createEstimate');
    reqObj.put('requestJSON', reqJSON);

//  Placing a request to Breadwinner Global API
Map<String, Object> resp = breadwinner_ns.BreadwinnerNetSuiteAPI.call(reqObj);
System.debug(resp);