You go to the hospital and check in with the stress test desk. You go to imaging and get a CT scan that lasts maybe 10-15 minutes. You lie on a shelf that moves you through a 'donut' made of metal and electronics several times. It's a bit noisy, so you might be given hearing protection. You are to go back to the stress test desk, you wait a bit, and then get invited into the lab, get an IV inserted, and an attending cardiologist or internist will drop by to invite you to begin. You step on a treadmill and begin to walk. After several minutes, they raise the front end to make more of an incline so that you're walking uphill, but they also speed up the treadmill. After several increments, you may have to stop, but they'll ask you to go until you simply can't any more, but not the point of passing out or endangering yourself by running out of leg speed and possibly falling. In fact, the physician will stop the test at some point when he/she has the data and/or your heart shows distinct signs of duress such as SVT/fibrillation, etc. Just before they think they should stop, or you do, they'll push the radioactive dye into the IV and make you go another minute or more. You get off, gasping and wheezing, they'll take your pulse and monitor you while you cool down and gather yourself. The attending will show you the results as he/she sees them with just the briefest of cursory analysis...first glance. Later you'll get a more detailed report after an expert has looked at your data. Back to the waiting room, they ask you to eat and drink a light lunch, and you wait for about half an hour...if I recall...it has been a few years. Back to Imaging for a second CT scan, but now with the radioactive dy in your system.

This is nothing to fear, except that you do get a big whack of radiation that day, or some go over two days. I have had two MIBI stress tests about three years apart. Looking back, I would ask them why not an MRI and/or an angiogram. Neither is radiation-free, but you won't be getting the equivalent of 500 chest X-rays like the nuclear stress test. Or, so my cardiologist ethically told me, and so did the handout he gave me to take home in preparation.
https://www.ottawaheart.ca/sites/default/files/legacy/uploads/documents/Patients-Visitors/tests-and-procedures/instructions-for-stress-myocardial-perfusion-test-2018-12.pdf