Open the active instance logs
Look in the failed profile's logs folder for latest.log, and check crash-reports if a report was generated. A launcher summary can omit the original dependency error. Copy relevant information before another launch replaces the latest session's log.
Find the first useful failure
Look for dependency checks, incompatible versions and the first relevant exception near the failure. Later errors can be consequences rather than causes. A library name in a stack trace indicates code involved in execution; it does not by itself prove that library introduced the problem.
Prepare a useful report
Include exact Minecraft, loader and mod versions, what you changed and steps that reproduce the failure. Share the requested report through the project's documented support route. Remove personal information, access tokens and private server details before uploading logs publicly.