You already own the tools you need to stretch your favorite pair of non-leather sandals. Though synthetic materials do not expand as easily as leather, you can extend your non-leather sandals without having to forcibly stretch them by hand or buy costly shoe stretchers. With just a hair dryer and an extra pair of socks or two, your non-leather sandals will be stretched and ready to wear comfortably.
Put a couple pairs of thick socks on your feet and firmly place your feet into the sandals. This may be a bit uncomfortable, but only momentarily as you stretch the shoes.
Aim a hair dryer at your sandals as you are wearing them. Blast the shoes with hot air, focusing on the areas that pinch the most.
Flex and stretch your feet while you have the hot air from the hair dryer aimed at the sandals. This will help loosen and stretch out the tight parts of the sandal.
Turn off the hair dryer after a couple of minutes and keep the shoes on while they cool.
Once your sandals have cooled, walk around in them barefoot to check the fit. If they still feel tight, repeat the steps above -- the shoes may need more drying time, or you may need to don thicker socks to help the stretching process.