Underworks is a brand of supportive undergarments. They offer binding tops to flatten the chest, called binders, that are useful for creating the illusion of a male chest on female bodies, or to control chest size for those with male bodies. These garments are made of a nylon/spandex fabric and have some stretch to them. You want them to be tight in order to bind your chest, but sometimes they are just a little too small and need a bit of stretching.
Underworks binders are all made of a fabric that is 70% nylon and 30% spandex. It is slightly stretchy, in order to get the binder on and off, and to move with your body comfortably. This synthetic material is harder to stretch on purpose than a simple t-shirt, since you cannot use heat on it without degrading the material. Using a hot iron to stretch and smooth the material, as you can with a cotton T-shirt, will ruin the binder.
How to Stretch
Binders are best stretched by wearing them to break them in. Once you get your binder on, you can wear it for eight to twelve hours at a time, since it is uncomfortable to bind all day. It will stretch with your body a slight amount. Repeated wear will stretch it to fit better, although eventually the stretching will reduce the binding power of the shirt. You can also stretch the binder by pulling it over the back of a chair and leaving it for a while.
Start with a binder that is properly sized. It is acceptable to need a little stretching for comfort, but binders that are too small can constrict you or even bruise the ribs--or cause breathing problems.
How to Get It On the First Time
You can't stretch a binder by wearing it if you can't get it on you. The first wearing is the most difficult. If you cannot get the binder on over your head like a shirt, pull it on. Step into it and pull it up over your hips, then put your arms in. This will eliminate the process of pulling it over your shoulders, which can sometimes trap your arms and make the process even more difficult.
If you still have difficulty, try putting baby powder on your body before putting on the binder. Never put on a binder while you are wet from showering or sweating. The moisture causes the material to cling to you.
Care and Cleaning
The way you care for your binder will also affect the fit. Binders should be washed in cold water and air dried. Hot water will make the fibers shrink, undoing all your stretching. Using a hot iron or machine dryer will degrade the fabric, as will using bleach. Air drying and avoiding harsh chemicals will keep your binder from being ruined, so you don't have to go through the first wearing and stretching too often.