You should start with dry, clean hair. You can use a conditioning spray before straightening. Make sure your hair is completely dry because hair that is even slightly damp will not respond well to straightening. Apart from this, using a hair straightener on damp hair will cause it to "cook" as the steaming causes the hair shaft to swell and forces the moisture out from the hairs cortex.
Make sure that the iron you are using for your hair is the right size for your hair length. Most women tend to use irons with heating plates that are 1" to 1 ½" wide although if you have very long hair then you can use wider irons. The smaller irons are much lighter and thus offer better control.
Separate your hair into small sections as trying to straighten chunks of hair will give you uneven results. In these cases, the hair on the inside does not straighten. By straightening smaller sections you will get better results. If you can use a little hairspray or mousse or gel on the straightened hair then you will be able to extend the life of your style.
You start off with a small section of hair as mentioned above. Start at the roots and slowly move the flat iron through your hair ensuring that the iron is at no point stationery. Each time you pass through the hair, comb it till your hair is smooth. Continue this process for each section of your hair until all of the hair on your head is straight.