It’s pretty easy really but very time consuming –
Strip with nitromors or get it chemical/alcohol dipped to remove the paint, if you use nitro use a brass wire brush to remove the bubbling paint.
If the metal hasn’t got any major scratching you could probably start with 400 grit or maybe even 600, you don’t need to go any higher than 800 though.
Next you either need a bench grinder/polisher or a drill along with a selection of mops and blocks of polish. A bike frame would probably be easier with a drill.
Start of with a sisal mop and a course polish and this will remove the sanding marks.
Then a closed stitch coloured mop with a medium polish which will remove any fine marks and colour the metal to an extent.
Then finish off with a loose stitch mop and a blue or green fine polish which will properly colour the metal and give you a mirror finish…if you’ve done all the other stages right.
When using the mops keep them moving as they can generate a lot of heat very quickly and it will give you a flatter/better finish.
Check out the polishing shop for all the different bits you need, I’d imagine £30-40 would get you up and running…