Internal tools often fail quietly. Nobody complains; people simply go back to the spreadsheet. Usually the tool was designed around the data model rather than the work.
Start by watching the task. Sit with the people who do it and note what they look at first, what they check twice and where they switch between screens. Those moments show what the interface should prioritise.
Design for the most frequent action, not every possible one. Keep the common path short, use the language the team already uses, and make errors easy to understand and undo.
Then release a small version early and observe it in use. Adoption is the real measure of an internal tool, and it is far easier to adjust a workflow in its first weeks than after habits have formed around it.