Step by step:

1- Lookup addresses of tokens you want to swap using the endpoint /tokens/{blockchainId}, for example WBNB ( 0xbb4cdb9cbd36b01bd1cbaebf2de08d9173bc095c ) -> BUSD ( 0xe9e7cea3dedca5984780bafc599bd69add087d56 ).

2- Give approval for the router to spend source token using the endpoint /approve/{blockchainId}.

3- Monitor the best exchange route using the endpoint /rate/{blockchainId}.

4- When you ready use to perform swap the endpoint /swap/{blockchainId}.

More info about the API response schema at https://crypto-exchange-app-server.herokuapp.com/api-docs