Guitar Nerds Anonymous

Fret rubbers look pretty simple to use. Crimson guitars do a set of you want to save yourself some import tax from stewmac.
https://www.crimsonguitars.com/collections/fretting/products/fret-rubber-fret-polishing-abrasive
There are loads of cheaper versions available on eBay but I’m not sure on the quality.

I've polished frets using fine wire wool, then hitting them with Autosol metal polish. However, you need to make sure that the whole fretboard is taped off so you don't get any of that on the wood.

I have a set of those and they're pretty good. I wish they'd made the colours more distinct though - it's nigh impossible to know the right order to work through. They're good for removing scratches in FitBit screens as well. But they do clog up and they're a bit pricey once you've imported them. Really fine sandpaper with cloth backing would probably do the same for a few quid.