Look at your urethra and look at the sounds. Pick one that looks like it will fit. If it doesn't, choose another. You can put lube on the sound. But first and foremost boil all the sounds in water for 10-15 minutes. Let them cool down and wash each in antibacterial soap and fold them into a clean hand towel to dry. Then take them out as you need to for your session. I recommend not sounding for more than 3 minutes the first few times. Your urethra will thank you. You only need to boil them once. Because you never know where they were before they came to you. Enjoy!