@J_123 I have had Mirena for just over 3 weeks now. I had the copper IUD for about 10 months before it.
Insertion can feel a little unpleasant, and some women find it more painful than others. Honestly though, the pain is just really bad menstrual cramps. The procedure only lasts for a couple of minutes so it's over before you know it.
They usually begin by putting a speculum inside your vagina, which opens the vagina walls so that they can see/access the cervix. That part usually feels like pressure, and can feel a bit uncomfortable, but generally is not the worst part. They usually take a couple of swabs to check for infection, and sometimes use a local anaesthetic to numb the cervix. They will insert a tube through your cervix which is usually the most crampy part. They check the size of your uterus to ensure the IUD will fit, and through that tube they push the IUD through until it reaches your uterus. Once that is done, they remove the tube, and may or may not trim the strings that hang through your cervix - they need to be left a certain length so that you can check them each month, and so the IUD is easy to remove when the time comes. Then they will remove the speculum and the job is done. All that takes about two minutes.
You can take painkillers 45 minutes before the procedure, and I would recommend you take the rest of the day off to rest. After my copper IUD procedure I cramped on and off quite badly for 2-3 days. A hot water bottle and painkillers help. I also spotted for 2-3 days.
After having the copper IUD removed , I had Mirena inserted. The procedure was very much the same as the first time in terms of pain. However, the minute the speculum was removed, I felt instantly normal and suffered no cramping at all (probably because my uterus was already used to something being there). I spotted for about a week, and haven't bled since. I have had some significant cramping over the last few days, and a bit of mild breast tenderness. But so far I am happy, as I hope the cramps will settle down as my body adjusts. Remember that everyone has a different experience, even during the insertion procedure. Some women experience barely any pain at all.