I use the thickest 16mm paint brush... the narrow end is 10mm thick and when it reaches the prostate the thickest part stretches my hole.. its amazing feel
Just slowly stretch it, it takes time. Use a brush that widens, where you can fit one side in but not the other, insert it to the point, where it matches your current max diameter, and the push it in. It will hurt (which is one of the reasons I am doing it), and you will be able to push it in a millimeter or two further, increasing the diameter just a little bit. Keep doing this over and over.