To derive monthly median values in Excel, first select the data that you want to use. Then, click on the "Data" tab and choose "Sort." In the Sort dialog box, choose "Month" as your sort order. This will ensure that all of your data is sorted by month. Next, click on the "Insert" tab and choose "Function." In the Function dialog box, choose "MEDIAN" as your function. This will insert the MEDIAN function into your spreadsheet. Finally, select the cells that you want to include in your monthly median calculation and press Enter. Your monthly median values will now be calculated!