When your bottom thread is messy it is the top thread needing adjustment. Your machine may not be threaded properly and the thread may not be sitting correctly between the tension discs. Check to make sure your machine is threaded correctly. Check thread between the tension discs. After checking, adjust the tension (tighter) and do a test. If it is still messy add some more tension and do another test.
Answers & Comments
Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Test your machine and adjust the tension as needed.
When your bottom thread is messy it is the top thread needing adjustment. Your machine may not be threaded properly and the thread may not be sitting correctly between the tension discs.
Check to make sure your machine is threaded correctly.
Check thread between the tension discs.
After checking, adjust the tension (tighter) and do a test. If it is still messy add some more tension and do another test.