It's easier with them off:
1979 R80/7 before and after:
My (now sold)1150GSA:
They can be done in place but it's a lot more fiddly.
Start with an abrasive but not too aggressive. 1200 grit wet&dry paper with lots of soapy water should remove the worst tarnishing but it will leave lots of tiny scratches that then need polishing out. Use medium steel wool (B&Q sell packs for about 3 quid) and Solvol. You may be able to move straight onto Solvol and a cloth but strips of strong cardboard with a squirt of Solvol is another good buffing method.