User Story Mapping is something that you probably have heard about if you work as product owner, business analyst or requirement analyst. It is a great tool for quickly break down customer journeys into system areas to map out work. The trick however is to use it where it is useful, which is in the business process when working with business analysis.
I often see people presenting User Story Mapping as a requirement tool, or even something that is useful for development as a backlog tool. This is usually suggested by business analysts and managers such as product owners, which makes sense because it is for them User Story Mapping is actually useful. Unfortunately it makes no sense for a developer since code are not following a customer journey.
For development, it is often very difficult to map things into a User Story Map. This is especially difficult for backend development where a lot of the work never even is seen by the end user. This causes some issues, not because User Story Mapping is bad in any way, but because it is defined on a user story level, when it is actually a process above the user story level.
For me this process is best used on the business side to map out features by the product owner and the Business Analyst. This is where the User Story Map truly shines, making it easy to understand where in the customer journey certain features live in a visual way. That is not to say that it has no value to a developer, quite the opposite. It is very useful to understand what feature you are working on and where it fit in the flow of things.
It is just not what is important for the development itself.
When it comes to the development itself you still want to have well-defined user stories in the form of work orders and acceptance criteria. Each user story also need to be small enough to be possible to complete within one increment (one sprint) and in most cases you will find that the user stories presented in the User Story Map are way too big for that.
I would actually suggest to not use the term user story mapping since that to me is misleading. I would instead call it User Feature Mapping to avoid confusion.
You can use this purely analog by mapping up a customer journey on a white board and then use that with stickies, or you can take advantage of apps in Jira for example. My two favorites are Easy Agile User Story Maps for Jira by Easy Agile and Agile User Story Mapping Board for Jira by DevSamurai. Both are great tools that will give you the tools you need to get started with User Story Mapping.