The best method is to use two large bar clamps and a pocket hole jig kit. First, be sure your mating surfaces are perfectly square, next mark the area where you will need to drill the pocket holes and drill them.
To assemble, lay the boards on a flat surface and apply glue to the edges that meet…next place your bar clamp on the work to hold it tightly. Finally insert the pocket screws and allow to dry. Makes for a very strong joint.
Here is the Kreg Master system for pocket hole joinery: http://www.rockler.com/kreg-jig-k4ms-master-system/?sid=AF523
and here is the cheaper Kreg R3 system: http://www.rockler.com/kreg-r3-pocket-hole-system/?sid=AF523
watch the product demo videos to learn how pocket hole joinery works.