Stair railings make exterior steps easier to climb and safer. For an informal exterior, you don't need to get fancy, but you do want a railing that is sturdy and durable. Building a simple outdoor stair   is a straightforward job that can be accomplished with a few simple tools and a basic knowledge of carpentry.
Instructions as follows

Firstly, cut back the stair tread on the highest and lowest step if they overhang the stair stringer. The tread is the flat part that you step on when you climb the stairs. The stringers are the diagonal boards underneath the stairs that hold up the treads. The easiest way to do this is to simply cut the whole tread flush with the stringer, but if you cut out a notch just large enough for a post, it will look better and make the post stronger.
Secondly, place a 4-by-4-inch post against the stringer at the bottom of the granite stairs. Drill two pilot holes through the post and into the stringer, then secure the post to the stringer with two lag bolts. If you can access the inside face of the stringer, you can also use flat bolts with washers and nuts, which will make the post even stronger.
Thirdly, attach a 4-by-4 post to the highest step in the same way that you attached one to the lowest step.
Fourthly, hold a 2-by-6-inch board up against the sides of the two posts, at the height where you want it when the railings are finished. This is easier if you have someone to help you. Trace a line across the sides of the post where they meet the bottom of the 2-by-6. Remove the 2-by-6, and cut off the tops of the posts along the lines you drew using a circular saw.