Welcome to StackOverflow! As written this question is too broad. A rough outline of how to do this (this got too long for a comment):
You need to get the companion guides from all involved trading partners and come up with a mapping from the accept/reject message to the 990 message, and then send it to your partner(s). This will involve having access to the original message data to make sure you correlate the response and route it to the correct partner. There are many many patterns to achieve this, and quite honestly many bad ways; there are also several finer points to it since it involves EDI. If you have a more specific question about messaging patterns involved here, it'd be better to ask about that in a separate question (with specific details about what pattern(s) you're trying and what issue(s) you're having, as well as reproducible examples).
Keep in mind that this is a non-trivial task, and any particular implementation of this task would involve localized factors that would make full details not well suited for StackOverflow. This question will likely end up being closed as such.