I've done them with a straight razor and it all comes down to technique and sharp blades. Sharp blades are most likely to cut hair than skin on a flat surface. Make sure to either use a new blade or strop your SR
Firstly cook your nuts, really steam your boys. You want them very warm and relaxed so you can gently pinch, stretch and wrap the loose skin around your finger. At that point it becomes much more accessible. I recommend doing this sitting. If using a straight razor use a steep, less aggressive angle.
For weed wacking around a post, pull it tight and if its too slippery, use a single slip of toilet paper to hold on. Once everything is in tension you may tread carefully with your razor(s). You may need to pull it in many directions to reach every spot.
That will get 50-70% of hair. For the rest I recommend squatting over a mirror. Again, make sure to pull everything in tension before hand and get everything very hot and steamy. Pubes are like spaghetti, they're softer when cooked.