Hey! I wrote this a while ago but figured it was useful here!
I highly suggest reading up and watching some Youtube videos before attempting any urethral play. Here are some tips that should help though:
Sterile vs. Clean
For catheter play it is best to stay as sterile as possible. Your body is naturally extremely good at keeping outside organisms outside of your urinary track, for good reason since this leads directly to your blood (through the kidneys). If you wanted to use sounds there would be a little more leeway since the risk of infection is significantly less. Note - for females - the sound may enter the bladder in which case it is important to follow full sterile technique.
Tips
-Use only sterile, new, unopened catheters.
-Buy them in kits (it might cost an extra $7 or so but it is well worth it, especially for the first couple).
-Use sterile technique - again, let clinical youtube (not xtube!) videos guide you here.
Gold Standard
The "gold standard" would be iodine followed by isopropanol/ethanol repeating each at least once. Most kits will come with 3 iodine wipes, it is important to move in circles away from the center and extending distally to anywhere you may touch with your sterile glove later. I normally will do the first couple in regular gloves before switching to the included sterile gloves. Another consideration is the "sterile field" - essentially the area you can touch your hands and the catheter. Anything you touch with your normal gloves or hands is considered contaminated, so when opening the kit you should be careful.
Risk Aware
Catheterization can result in injuries, infections and potentially death (is this sounding like a drug commercial yet?). It is important that you be willing to seek medical attention promptly should something go wrong. If you aren't sure if you could do this, I don't suggest you do this kind of play.
Have fun!
Done correctly urethral play can provide the most amazing experiences. The control the "doctor" has over the patient's body is absolute and the sensation is as hard to explain as an orgasm.