Which software do you have? If it's VCarve or Aspire then the fluting toolpath is the way to go.
With Cut2D there aren't any 2.5d toolpaths like fluting and v-carving so you'll need to get creative. One way is to create a series of straight lines on top of each other with each one being shorter than the previous one. You can then create a profile toolpath for each one with sequential start depths and the same cut depths to produce a sloped slot.