Here's the tricky bit.
A sheet obviously can't support itself, so you need to come up with a means of supporting it. Preferably, this can just be a friend of yours with a bit of height on their side, which is both easily adjustable with a little communication, and mobile, as your friend will likely have legs that they can walk around with. Otherwise, you might need to get creative with strings and existent supports or tripods.
The beauty of using a sheet is that it doubles both as a reflector and a diffuser, as well as being a handy background. Vary the positioning of the subject relative to the sheet to use it in its different capacities.
As a reflector, the sheet should be situated such that it will bounce the light back at the model at the desired angle. The model could sit or otherwise pose upon it, for instance, so that the light bounces up at their face and eliminates unsightly wrinkles. Or, if you position the model in the shade, the light could be reflected from a patch of sun onto the model's face and thus bring in some much needed dynamism.
As a diffuser, the sheet needs to be placed directly between the model and the light. This will soften the harsh contrast between sunlight and shadow that occurs on a sunny day, and will create a generally more pleasing picture. Different weaves will diffuse the light differently, so you may want to try a few different sheets out to get the desired effect.
As a background, the sheet will provide a wonderful glow to the image, as well as providing a simple, minimalist background with beautiful natural lighting. Be careful to also reflect/diffuse light into the model's face if you do this, creating a close semicircle around the model, as otherwise it may appear too dark relative to the background.