When it comes to wearing a promise ring, it is important because of what it symbolizes, not necessarily how it is worn. There are many different options for how to wear your promise ring - whether that be on your hand or around your neck on a chain, like a necklace. How you wear your promise ring depends on personal preference, and there are many options to choose from.
First, the most common place that people wear a promise ring is on their left ring finger. Usually, a promise ring is a promise of commitment and often (not always) precedes an engagement. Therefore, many couples like to wear a promise ring on what is traditionally thought of as being the wedding ring finger: a.k.a. the ring finger of the left hand.Something to remember is that every finger is usually a different size - the ring finger of a left hand may be a different size to the ring finger on the right hand. If a promise ring is going to be an expensive purchase, it is worth considering whether the size of the ring can be altered if it will be worn differently in the future.
Second, is the ring finger on the right hand. This leaves the ring finger on the left hand open, in hopes of receiving an engagement ring in the future. This is a good solution that gets around any concerns about needing to get a ring resized - as this is usually wear a promise ring goes once engaged. This also helps to avoid awkward questions whether or not the couple is engaged - not that it is any of their business anyway!
Other ways to wear a promise ring is on other fingers - some people choose to do a pinky promise ring, which is worn on the pinky finger and symbolizes a pinky promise to each other. You could also wear it as a pendant around your neck, or as a charm on a charm bracelet. Some even choose something alternative to a ring, such as other pieces of jewelry or something special that will remind each other on a daily basis of their commitment.
Overall, when it comes to deciding how you should wear a promise ring, it is important to remember that there are no rules. You can wear it however you wish! The options listed above are just ideas of what most people do. The important part is the commitment that you and your partner are making to each other, not what finger you wear it on or wherever else you decide to wear the ring!