Here is an answer to the question How do you build deck stairs.
Plan everything out on paper first. The height of all your steps needs to be the same. Tread depth should be 10″ or more. Rise height should not exceed 7 1/2″nor be less than 4″. Your stairway should be at least 36″ wide. You will also need a level landing at least 36″ deep (which seems excessive, but I didn’t write the code). Stringers should be between 12″ and 16″ apart. Armed with all that information plus the height of your stairway, you can figure out the exact rise height, number of steps, number of stringers, and overall length of your stairway.
For stringers, I start with a treated 2×12 of appropriate length. Mark notches in chalk because it’s easy to see on dark wood. Cut mostly with a circular saw and finish up each cut with a Sawzall. Stringers get attached with galvanized hangers top side and set solidly on concrete footings or a lower deck on the bottom.
Treads and risers should be the same material as the rest of your deck. Ipe, redwood, or Trex in my neck of the woods, but may vary at your location.
Railings are outside the scope of the question.