When you're done, remove the flanges, pull the top layer of cloth down, and make sure your nipples aren't directly behind the slit. Go back to work, ...
If you've got a baby, and work outside of the home, or otherwise need to be away from your kid from time to time, you're probably already acquainted with breast pumps. While being able to produce tasty, nutritious baby food with one appliance, and body parts you already had on hand is a good thing, it's not always convenient. Being able to easily eat, type, or read while pumping makes the task a lot more pleasant, and making a hands-free setup is simple. There are bands and bustiers you can buy for this purpose, but they need to be used over nursing bras, which I prefer not to wear, and putting them on is an extra step.
Buy or find a full-coverage bra with formed cups. Basically, what you want is something with firm, foam cups with a layer of fabric over those. A lot of 't-shirt bras' would work, and a lace overlay would be good, too. It's just important that there be two layers, and the bottom one needs some structure. Find some scissors, too.
Slit the fabric overlay along the bottom, so you can pull it up and over the cups.
Put the bra on, and mark the spots where your nipples fall, either with a pencil, or just by pinching the fabric. Remove it, and cut a vertical slit either through this mark, or just a bit to the side of it, towards the outside of the cup. Just cut enough so you can fit your pump's flanges through the slit.
Make sure your pump flanges fit, put the bra on, and you're good to go. You can adjust yourself to make sure everything's lined up right. I usually put the bra on, and then fit the flanges into it.