Clone this repository and run: ./monerochan-merchant-rpc in this directory.
Afterwards you can visit the interactive docs page at http://localhost:3001/api-docs/
You can interactively play around with the api calls in the docs page. The curl commands generated by this doc page can be executed directly in the page (no need to copy them.) or you can convert them automatically to the language of your choice. For example this is a tool that you can use if you want to use this api with ruby: https://jhawthorn.github.io/curl-to-ruby/
Please also take a look at the monero-playground In this repo you will find everything that is necessary to setup a local monero testnet. (You will need it if you want to play around with this API and you want immedate responses instead of the added latency of using stagenet nodes). It can be done very easily by just cloning the repo and running two commands.
The page you can access now in your browser at http://localhost:3001/api-docs/ looks like this:
You wont be able to make invoices while the wallet is not fully synced. Use the status endpoint to verify that the wallet is synced first. After that You can use the makeinvoice endpoint.