First, you try to understand what the formulas express. In the example your mentioned, the second formula is already valid. That makes it a bit easier. If the formula is valid, you can only satisfy it. Thus, it just remains to find an example that doesn't satisfy the first formula. The formula requires either a to hold on every step, or ¬a to hold on every step. Now we just need to produce a path where that's not the case.
