Please consult http://labelary.com/zpl.html for general information about ZPL.
To test your ZPL label, use the API to create the label with an order and export as a ZPL file. Using the file, copy the information in this ZPL reader BinaryKits.Zpl.Viewer.WebApi (binarykits-zpl-viewer.azurewebsites.net) to make sure that the label is valid and comes out as you wanted.