Here's a few different ways
There is a good tutorial here:
Immerse in an acid of some kind for a while. Lemon, Vinegar, Bleach etc.......
Or you could fake it (fine spray or use a dry brush using rust coloured paint.)
Here’s my trick: https://www.frugalfamilytimes.com/2016/10/the-secret-ingredient-for-realistic.html?m=1
let them soak in lemon juice or vinegar overnight.
If they are really metal and not plastic bells, here's a video for you....
Hello I would consider using spray paints to faux finish. You can have some remarkable results and patina without having rust particles I could rub off and stain things
This is what I do.
Put bells in a large glass jar. Do not fill more than halfway. Pour in regular household bleach to cover. Let sit at least 24 hours. Discard bleach down drain.
Cover bells with vinegar and a couple Tbsp of salt. Shake gently. Let sit for at least 24 hrs. Discard cider down drain. Allow to air dry at least 24 hours.
To air dry I place them on several layers of newspaper.