The stair stringers are essentially the backbone of your staircase. Always use straight high quality pressure treated 2x12’s for stair stringers. Make sure the wood has .60 RET level if you are planning on resting the stringer on the ground.
In order to prepare the stringers you will want to lay out one of your 2x12’s on saw horses and use a framing square and clamps to trace the outline of the stair using the information from the decks.com stair calculator. If you line up the riser height and tread depth on the framing square and align it to the stringer you will be able to find the exact angle to cut your stairs. Repeat this process along the stringer. Because the bottom step will be the ground you will find that it will be too short by the height of one tread. After the stringers have been marked you must make an adjustment. Drop the stringer by cutting off the thickness of one tread to adjust for landing on the ground. Depending on how you are attaching the top step you may need to make an additional adjustment to maintain a uniform stair design.
Use a circular saw to perform the cuts but do not over cut your guidelines. This will look bad and will weaken the stringer. Instead use a handsaw to finish the cut cleanly. Once one stringer has been completed and tested for accuracy you can simply use it as a template to produce the others.