Your Canon dSLR has a useful tool on its strap that has likely gone unnoticed. PetaPixel points out that the small rubber thingy, (for lack of a better term) on the strap that came with your Canon camera is an eyepiece cover that prevents light from leaking into the viewfinder.
Whenever you don't use the viewfinder to shoot a photo -- when using the self-timer or remote switch, for example -- light can enter the viewfinder and throw off the exposure. To prevent that from happening, just mask the viewfinder with the rubber eyepiece cover.
I use an old Nikon D50 and have no such rubber thingy on the strap. According to my D50 user manual, an eyepiece cover was included in the box. Since it wasn't cleverly integrated onto the strap, I lost it years ago.
I shoot with a Nikon, so I went to my local Best Buy and asked a patient store associate to open a Canon dSLR box. The included strap had the eyepiece cover that my Nikon lacks. To use the eyepiece cover, I had to remove the rubber eyepiece cup from the back of the camera and then slipped the cover over the viewfinder.
If you need to cover the viewfinder and don't have a rubber eyepiece cover handy -- Lori Grunin, one of CNET's resident photography experts, suggests you simply drape a black microfiber cloth over the viewfinder.
For more photo tips, learn how to protect your camera from rain and snow for less than $1 and how to use everyday items as DIY phone camera lenses and filters.