I’m assuming you’re using Signlab.
For example if printing red as a spot colour – ensure your file is painted a specific red colour (i.e the first red you come to in the default palette). Then when you go to print select the red spot colour in the windows print manager. Ensure you have a spot colour cartridge inserted that has the red bar code. You don’t need to remove any other cartidges and if the red cartidge is not installed the machine will beep and wait for you to fit the correct spot colour
You can only print spot colours with vector drawn images. Rendered images (i.e JPG BMP etc) will print CMYK even though the colour may look the same shade of red for example