Before you upload your stl file make sure your file is in the right orientation. If it is not you can fix the problem with a stl viewer and export the stl again in the right orientation.
When you succesfully uploaded your 3d model you need to select the material, nozzle size and layer height. On the extra tab you can select if you want to use supports and if you want them removed after the print.
When the previous steps were succesfull you can now order your print. Start by adding the configured model to cart by clicking on the button "Add to cart". If you need more than 1 model you can now configure another model. When you're done click on the cart icon. To complete your order click on the button "Order" than you need to fill in your personal info. Now you succesfully ordered your part(s).