It can be tricky to fit stringers to a poured concrete slab landing. For one thing, the slabs are always curved at their edges, making it difficult to get bearing on both the toe and heel of the stringer. Another problem is that the only way to positively connect the stringers to the landing is to first anchor a 2×4 cleat to the slab, notch the stringers around that cleat, and then toenail the stringers to the cleat. So I anchor my stringers to a pair of ground-contact-rated PT 6x6s cut to the width of the stairs, as shown in the drawing.
I start by determining where the steps will end on the ground, then excavate another 5 ½ inches to create a level area for the 6x6s. Leaving a couple of extra inches all around allows for some wiggle room to make adjustments later on. Before placing the 6x6s on the ground, I notch one of them to accommodate my 4×4 guard posts.
Getting the 6x6s set so that they are nice and level can take a little time, but it’s important that the entire base of each stringer has full bearing. Once all the stringers are sitting properly on the 6x6s, I fasten everything together with structural screws, then backfill around the assembly. I’m always surprised by how strong the railing posts get, especially after the finished skirt boards lap over the posts and the first riser is installed.