Install a screenwriting program; Final Draft is the best. Open a new document. Hit enter and select the "transition" key. Type in your transition; fade-in, dissolve to, over black, etc. Then hit enter and type your slug line, which states interior or exterior, location and the time of day. Ex. INT. PETE'S HOUSE - NIGHT.
Go to the next line and write the action lines, which describe the action taking place as well as introducing the characters. The action lines should be written in the present tense. No -ing words. The characters' names should be in capital letters when first introduced but in regular fashion afterward.
Write dialogue by first typing the character's name in capital letters in the center of the page. The dialogue follows under the character's name with the spacing already given by the program.
Continue to type out each scene in this fashion. You'll establish slug lines, action lines, character names and dialogue. Go scene after scene until you have a complete screenplay anywhere from 105-120 pages.
Print out your screenplay and use a three-hole punch to place in order. Place two brads in the script, one through the top hole and one through the bottom hole. Press the back of the brads down so the script is secure. That's it. You have a perfectly formatted script.