Goedjn wrote:

Mortises. Cut a 2" deep hole exactly the shape of the rail
in the posts against the wall. Cut an identical hole
all the way through the posts flanking the stairs.
Slide the railing through and put a screw down from
the top of the posts, and cover it with a post-cap.

Cutting a through mortise where the exposed end is facing the stairs?

What the hell is with post 4?

It's not installed yet. He leaned it up against the house so you'd
get the idea.

To the OP: There are many ways to do it, some more attractive or
easier to do than others. In your situation I'd probably use some
railing brackets on the lower rail (since you don't have the clearance
from below and toe-screws from above don't look that great), and toe-
screw from underneath the upper rail. You local lumber yard or a big
box store will have rail brackets for vinyl or composite railings and
you can use those.

R