Create

//  Create a Map to store all the data of an Item 1
Map<String, Object> nsItem1 = new Map<String, Object>();
    nsItem1.put('itemType', 'nonInventoryResaleItem');
    nsItem1.put('itemId', 'Leather Jacket');
    nsItem1.put('taxSchedule', new Map<String, Object>{'internalId'=>'1'});
    nsItem1.put('incomeAccount', new Map<String, Object>{'internalId'=>'54'});
    nsItem1.put('expenseAccount', new Map<String, Object>{'internalId'=>'58'});
    nsItem1.put('salesDescription', 'Blue Color Leather Jacket');

    //  Provide the Subsidiary List   
    List<Object> subsidiaryList1 = new List<Object>{new Map<String, Object>{'internalId'=>'1'}};
    nsItem1.put('subsidiaryList', new Map<String, Object>{'recordRef'=>subsidiaryList1});
    nsItem1.put('includeChildren', TRUE);

    //  Initializing the Item Custom Fields
    List<Object> itemCustomFieldList = new List<Object>();
    Map<String, Object> itemCustomField = new Map<String, Object>();
        itemCustomField.put('fieldType', 'select');
        itemCustomField.put('scriptId', 'custitem_standardlist');
        itemCustomField.put('valueLookup', new Map<String, Object>{'internalId'=>'2'});
        itemCustomFieldList.add(itemCustomField);
    Map<String, Object> itemCustomField1 = new Map<String, Object>();
        itemCustomField1.put('fieldType', 'boolean');
        itemCustomField1.put('scriptId', 'custitem_checkbox');
        itemCustomField1.put('value','true');
        itemCustomFieldList.add(itemCustomField1);

//  Adding the Item Custom Fields
    nsItem1.put('customFieldList', new Map<String, Object>{'customField'=>itemCustomFieldList});

//  Create a Map to store all the data of an Item 2
Map<String, Object> nsItem2 = new Map<String, Object>();
    nsItem2.put('itemType', 'nonInventorySaleItem');
    nsItem2.put('itemId', 'Laptop');
    nsItem2.put('taxSchedule', new Map<String, Object>{'internalId'=>'1'});

//  Provide the Subsidiary List   
    List<Object> subsidiaryList2 = new List<Object>{new Map<String, Object>{'internalId'=>'1'}};
    nsItem2.put('subsidiaryList', new Map<String, Object>{'recordRef'=>subsidiaryList2});


//  Initializing the RequestJSON Data to be passed.
Map<String, Object> requestJSONMap = new Map<String, Object>();
    requestJSONMap.put('items', new List<Object>{nsItem1, nsItem2}); // similarly we can upto 20 items to the list 
    String reqJSON = JSON.serialize(requestJSONMap);

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

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