JIEM is a modeling tool to help facilitate the definition of information exchanges--getting the stakeholders who initiate the exchange together with those who receive it and then working out the detailed business rules and the data sets needed. The output of the JIEM tool is the documentation of this process that defines the exchange that is needed and what data should be there.
NIEM itself is a data model containing data component definitions and structure to help you build XML schemas to implement an exchange. The output of the NIEM process is an Information Exchange Package Documentation which is really a complete spec that can be given to a programmer to implement. The IEPD includes the XML schema, a sample instance and the business rules for initiating the exchange.
From a workflow perspective, the effort in the process of using the JIEM tool generates a practitioner friendly definition of a specific exchange. The next step is to develop a NIEM conformant IEPD that generates a technology friendly specification that a developer can turn into reality with programming.
It is not necessary to use JIEM for developing a NIEM-conformant exchange. Jurisdictions that have used the JEIM tool have found it very helpful in getting the practitioners on both sides of an exchange to better understand each other, but this can be done in other ways as well.