Ebony stain

First, clean the wood with a cleaner containing oxygen bleach (touted as non-toxic for plants, pets, and people). You will probably need to scrub a bit with a brush with stiff bristles. Make sure to test an area first. Then, let the posts dry for a few days, then sand as needed and stain. Any big box home improvement store will have all the supplies you need.

Thank you!!! I think we’re hung up in the “sand as needed”. Does it need to be super smooth? What’s the purpose of the sanding?

If you sand you open the pores so it can take stain/poly better. That post looks rough sawn so if you want to maintain that look one wouldn't sand a ton.