Identify the exact failure
Separate a launch crash from a world-loading error or an unexpected result during play. Note the last successful setup and the files changed since then. Save the complete log before starting another session that might replace it.
Verify the environment
Compare the Minecraft version, loader name and Clumps filename against the official file details. Check the active instance directory and look for duplicate Clumps jars. Read any dependency errors in the log and follow the requirements for the selected file.
Make a useful report
If the cause remains unclear, reproduce it in a test instance with the smallest practical set of mods. Include steps, expected behavior, actual behavior and the complete error log in a report to the project's issue tracker. Remove passwords or private server details before sharing logs. A repeatable example helps someone investigate much more effectively than a screenshot of the final crash message alone.
Next step: read the installation guide or find a matching download.
Project reference: Official Clumps project and file details.