Start with catheters and do yourself first so you know what it feels like. Only use sterile instruments and lube. Use betadine to clean the vaginal area or penis before insertion. Men are harder to sound and catheterize than women because the urethra is longer and curved.
In women, the urethra is short and straight, about 1 1/2 to 2". In men it is curved and 8-16" depending on how long the penis is, if it is erect, etc. Use about a 16 Fr catheter. If you want, later you can go up in size, one size at a time. Use a plain catheter, NOT a foley that you inflate, until you are experienced and really know what you are doing.
WARNING--Don't fuck up your women's urethra. You could make her incontinent for life or require surgery to correct. Either way, you will be toast. No one will be happy if you cause an infection either.
OK, I went through my files and I have posted this before, but can't find where, so here it is again:
If you use an appropriately sized catheter, plenty of lubricant and know how to do it, it is not painful. I don't have any pain when the catheter is inserted, and my husband will tell you it feels good, and he has described the sensation as "an orgasm in reverse" as the catheter goes through the prostatic portion of the urethra. There are also nerve endings throughout the urethra, especially in the male, which produce very pleasurable sensations. You and he will know you are in the prostatic urethra because there is an increase in resistance as you insert the catheter, followed by a decrease in resistance as it slips into the bladder. The catheter will go in about 8-10 inches (when flaccid) depending on the length of the penis and whether or not is erect when it is inserted. Obviously this length will be somewhat more if he is erect.
Stuff about pain: We generally hear horror stories about how catheterization hurts like hell. No one comes back from their physician and says it was no big deal. There are two good reasons for this. The first is no one wants to be labeled a perv by saying it was pleasurable (people like sympathy points) and second, most people having it done by their physician typically have a bladder or urethral infection, or some other problem, and then it will hurt. A healthy bladder and urethra will not have pain when a catheter is properly inserted. That’s why people like doing it!
Here are the caveats. Heed them:
Clean your hands (gloves are good) and the penis thoroughly with soap and water, then swab with betadine over the head of the penis. For yourself, clean your partner’s hands, your hands and vulva, your hair too if not shaved) and swab with betadine.
For a man you have never catheterized before, a 16 Fr sized catheter is the best starting point. If that goes in very easily and you want to go larger, go to 18 Fr, then 20 Fr and so on. Do not jump sizes. Do not assume that just because the penis is large that the urethra will also be wide in diameter. If you start too large, you may cause irritation and/or bleeding, and unless you and your partner are into blood (some people are, but this is not a good part of the anatomy for blood sports) this might be the last time your man will allow you to do this.
For yourself, a good starting point would also be the 16-18 French size. The female urethra is slightly larger in diameter than the male. To insert the catheter, lubricate the orifice of the urethra with a blob KY and coat the end couple of inches of the catheter. You’ll feel the tip enter the orifice and slowly insert it. You will feel a little resistance increase as you pass through the sphincter, then it will slip right into the bladder. The female urethra is only about an inch and a half or so long, so you’ll get right in. Have a bowl or container of sorts ready, because you will have a “champagne” fountain of urine to collect.
For your man, after cleaning with soap and water and betadine, place a blob of KY on the tip of the penis (no such thing as too much lube) and the end few inches of the catheter. Slowly insert the catheter. Here is where it can be tricky, after it goes in the length of the penis, lift the penis up into the position it takes when it becomes erect, because that will straighten out the urethra and it will go in much more smoothly and with less irritation than if you kept the penis down. After it is in about 8-12 inches or so, depending on the length of the penis, you will feel the resistance increase as you get to the prostatic portion. He will be able to tell you it is there, (that’s the reverse orgasm sensation) then after another inch or so, you’ll be in the bladder, so have a container ready for the flow!
If you need to add more lube to a catheter, go ahead. DO NOT USE SALIVA. It will introduce mouth bacteria to areas that never experience them, and infection will result
After this kind of play, especially if you have "virgin urethras" you will probably notice a little burn afterwards for a day or two when you urinate. This is normal and doesn't mean anything is wrong. If you become experienced with urethral play--catheters and sounds and the like, you won't have that sensation when you pee afterwards. If burning persists, you might need an antibiotic. This type of play is not without its risks.
Insertion of sounds is the same as catheters with one great exception: Catheters are flexible and will follow the curves inside the anatomy. A sound is a rigid piece of metal. In women the sounds are easily inserted into the bladder because the urethra is short and straight. Some types of sounds are straight and they will NOT make it into the bladder in men because the urethra is curved like a "J" as you go underneath the scrotal area and curve back up towards the bladder. Other sounds are "J" shaped, and with careful practice you will learn to introduce them into the bladder. After passing through the penis and passing the tip downward, you will need to direct the sound around the curve by taking the handle of the sound and pulling it away from the abdominal area so the tip follows the urethral path back up to the bladder. Again, as with the catheter, as you reach the prostatic portion of the urethra, you and he will feel the sound meet some resistance and then slide into the bladder.
Have fun, be careful, stop if there is real resistance and be good to one another.