When you get the mirena put in, the procedure is as follows. (or at least how it was for me)
When you are at home, you take 3 ibprofen and arrange for someone to drive you to the doc and back home.
When you get there, The doc tests you for pregnancy and std's. If you have std's, you will not get a mirena until it is gone because of the risk of pid.
The doc lectures you about all of the risks (scary part, but don't let it scare you too much, it's just a legal thing) and you have to sign stuff saying you are ok with it. Then you have to sign a hospital minor surgery form.
You put on your paper outfit and lay down. The doc opens you up just like during your pelvic exam. She is very nice but you are scared, and there is also the assisting nurse in the room.
The doc inserts a ruler (obviously it's a special ruler, not the regular grade school kind) through your cervix into your uterus to make sure that your uterus is at least 6 cm long. If it is not, you will not get the mirena. If you are me, this hurts a lot. A lot. But it is really quick.
Then the doc inserts the mirena through your cervix into your uterus. This feels slightly worse than the ruler did. But again, it is quick.
The doc cuts the strings (because they start out really long). They are like fishing line. The doc tell you how to feel for them and may let you feel the cut ones so you know what you are feeling for.
The doc goes to get your husband and instructs you to sit for 15 minutes so you don't faint when you get up. She also leaves a pad in the room for you because you might have some bleeding right away. (Bring one in case your doc isn't this thoughtful.)
She comes and checks on you 15 minutes later and you leave. On your way out, you schedule a follow up exam where she will just take a peek and maybe cut the strings shorter if they are bothering your husband.
I'd suggest at least taking the day off of work, or be prepared to load up on painkillers.
If you are me, you never get a period from that day on.
If you are me, in the next 5 months, you gain 20 lbs, relive your teenage acne problems, and fight a lot with your husband. And cry a lot.
If you are me, 9 months later you realize you cannot take the debilitating cramps any longer and you get that evil thing removed from you.
**Most people find that they have less side effects with the mirena because the hormones are more localized. You will not know if you will have side effects until you try it. I represent the worst case scenario, really.
My doctor told me that if the iud did perforate my uterus, I wouldn't feel it. It would cause problems and I may start having other symptoms, but I wouldn't actually feel it perforate (the uterine wall doesn't have many nerves, I guess?) The way to tell is if the strings disappeared. Check the strings when you have your period. There is a risk that it may fall out within the first couple of days especially if you haven't had children (although I can't imagine how feeling how it went it) so check it often in the first month.
You have to reach really deep to find the strings and sometimes they tuck themselves between your cervix and vaginal wall (you cervix kind've pokes out into your vagina and forms a crevice around it), so you might have to dig around.
I hope this helps. Good luck.