Summarize:

Use a flexible tape measure, and measure the length and width of your sofa from back to front and side to side. Mark down the highest and widest points, as that is the minimum size that you'll need to match to properly cover your sofa. Lots of shops sell covers based mainly off of the length of the couch, but depending on how customized you want the cover to be, it'll be helpful to have all the measurements before you start shopping. From solid colors to patterns, canvas material to cotton, and upscale versus casual, there are so many options to choose from when you decide to buy a new slipcover. Think about the other design elements present in the room and try to pick a cover that will fit in well. Many home goods stores carry slipcovers, and there are also thousands of options to choose from online. You can buy a slipcover for anywhere from $50 to $500, just depending on where you buy it from. Some companies offer to make custom slipcovers for their customers. If you're concerned about the fit of a one-size-fits-all type of situation, or if your couch is uniquely shaped, this might be a great option for you. Chances are your new slipcover is clean, but it probably also came into contact with some chemicals during the manufacturing process. Always check the care instructions before washing your slipcover; in general, it's typical to wash a slipcover in cool water on the gentle cycle and to then hang it up to dry. Sometimes the packaging that a slipcover comes in might have an odd smell, and washing the slipcover can help remove that scent from it.
Measure your sofa to make sure you're buying the right size cover. Shop for a color, fabric, and style that you like. Order a custom-made slipcover for a more professional look. Wash your slipcover when you first get it to remove any irritants.