The best way is to get a reversing ring that has male threads on both sides to "marry" your lenses together. You can also hand hold the fronts together. Put your camera/lens in manual focus mode and set the aperture of your 50mm to 1.8. The depth of field will be very shallow as you move in and out and a speedlight or flash will help with the keeper ratio.
If you have further questions, this video tutorial my help.