How do I stain an old concrete bench?

Masonry stain is the way to go.

https://www.hgtv.com/design/outdoor-design/outdoor-spaces/how-to-stain-concrete

https://www.doityourself.com/stry/how-to-paint-a-concrete-bench

If you want the details to stand out after staining paint some black or dark colored paint in the details and wipe the top off with a rag. The darker color will remain in the lowest part of the details. I did that similar thing on my cedar chest. It really made the carvings stand out.

Hi Karen: Here's a site that might help: https://www.concretecamouflage.com/how_to_stain_old_concrete.cfm

Good luck

Hi, You could use Fence Stain or Concrete stain will last a bit longer and paint should last even longer if well prep'd. Highlight the design by using a Contrasting

colour on the top only of the base colour. Hope that's of help to you.