How hard can laundry be, right? You might be surprised, if you're used to American coin-op laundromats.
The first thing you need to know is that Paris laundromats come with a choice of machine sizes. Some have 3, but most seem to hold two: 7kg and 10kg load sizes. You probably don't know how much your laundry weighs, so a lot of laundromats post descriptions of what the average load size might contain. Frankly, I don't think you need them. If the washer will comfortably hold your clothing, you're good. If it won't, you need a larger washer. Don't pack your clothes into the drum while congratulating yourself on saving a few euros. If your clothing won't fit inside without Herculean effort, trade up to a larger machine.
Once you've put your clothing into the washer, close the door and make sure it clicks and locks. If you're not sure you did it right and want to open it again, just push the button that says "porte," and the door will open. Choose your laundry temperature, but be aware, the temps are listed in Celsius, not Fahrenheit. Whites are the hottest temperatures, followed by colors, synthetics, nylon, and something called laine. If you want cold, or mostly cold, water, choose one of the last two.
Your detergent goes in the top. For most loads, you need about 300 cc's of powder or 1/2 a cup. You might save yourself some effort by buying detergent tabs. A normal washload will use 2 or 3 of the small ones. French detergent boxes are fairly confusing. Even the French find them so, and just end up eyeballing their detergent. If you don't have your own, there's generally a vending machine that sells loose powder. You'll need a cup to catch it, and there's no choice of brands. There are two places to put soap and a place for fabric softener in the little hatch in the top of the machine. One of the big spaces is for the prewash cycle, and the other is for the regular wash. There's usually a chart on the wall to tell you which is which and which washer settings get prewashes. Generally, both white (blanc) and color (coleur) settings have prewashes, while the others don't.
Paris laundromats have, for the most part, gone "hi-tech," so rather than dropping your money into the machine itself, you put it into a central box that controls all the machines at the location. Gone are the days of hoarding quarters; the control box will accept both coins and bills. Once everything is set and you're ready to go, you'll find the control box on one of the walls, usually in a corner. It's about 3.5 x 2.5 feet and has a digital LCD, coin slot, bill reader, and a coin return hatch. Enter the nunmber of the machine you're using. You'll find that printed on the front of the machine in big digits - generally plain black ones for smaller capacity washers and white ones inside a colored circle for larger capacity machines. When you've entered the number of your machine, the digital LCD will tell you how much money you owe. Feed it your bill or the correct amount of change, and you're good to go; your washer will start automatically and click off when it's done. If you forgot to add something to your washer, at this point either you're out of luck or it's going in through the top dispenser.
When you're finished washing, dryers work the same way and are generally 1 euro for 10 minutes, making drying a pretty expensive proposition. Save the cash except for heavy things like jeans and towels, and buy yourself a sèchoir à linge - a drying rack - at Carrefour. A small one will hold most of the stuff a single person will have in one load and costs around 15 euros. For 10 euros more, you can get one with about 7 square meters more space, but if you have a tiny apartment, you'll never be able to use it. Sèchoirs are great; most of the time your laundry will dry in just a few hours, but on a really humid day, you should allow for overnight.
Happy laundering!