When building deck stairs you will need to determine the total height of the deck at the stairs. This is the change in elevation from the top of the deck to where you expect the stairs to land. Place a level straight out from the deck and measure to the ground or landing using a tape measurer.
You will also need to establish the tread depth and riser height for each step. The minimum tread depth is 10" and the maximum riser height is 8". In most cases this will be a 7-1/2" rise and 10-1/2" run. This will allow you to use two deck boards for each tread with a 1/2" reveal.
Once you have determined the total stair height, tread depth, and riser height you can plug the numbers into the decks.com stair stringer calculator to find the stringer length, stair projection, and the angle of the stairs.