This is probably the best method to straighten your hair without using your flat iron.
Take a shower.
Brush your hair, removing all knots and tangles.
Add any desired products to your hair. Part your hair as you normally would, then split your hair in half.
On one side, take your brush, and starting at the top, brush your hair down pushing really hard so your hair is really flat and bobby pin it about 2 inches down from your hairline.
Do this all the way down, putting bobby pins wherever is necessary that will keep your hair flat and tight against your scalp.
Do the same thing with the other side. If your hair is long, you just want to bring your ends back up and pin them on your head.
All your hair should be tightly pinned against your head with a bunch of bobby pins.
Put a wig net on (those lunch lady hair things) and go to sleep on it or wait until your hair dries.
When it's fully dry, take the bobby pins out, and touch it up with a straightener if you'd like