Each spool contains a limited amount of thread, so use it efficiently. Some blocks in the puzzle have numbers on them (e.g., "2" or "1"), which means you need to make a match next to them that many times to clear them.
A useful trick is to create long chains of dots to generate spools with more thread. Prioritize creating spools for the colors you need most to complete the pattern. Strategically clearing the puzzle grid allows new dots to fall, potentially creating better opportunities for longer merges.