Yeah I'm installing two of the white balusters into that open space on the tread. There's already holes in the tread and railing for the baluster to sit. I'll glue the bottom of them in because they are doweled, but the top part is skinny and there is a lot of play where the railing hole is, so it needs to be nailed in.

I can post a better pic if you need. The ones I pulled out that needed to be replaced were nailed in, and looked like 18ga nails but no idea if that's actually the best practice. This house has lots of short cuts done to it I'm finding. I need these to be really secure because I have a child and last thing I want is here to grab or shake these and they somehow came loose.