Tighten until it feels "snug", but don't go any further. As long as it can't fall off, it's fine.
If the thermal paste is drooping over the sides when squished, then yes, you probably put a tad too much. Just wipe it away and it should be fine.
Ideally, you want to be drawing an 'X' across the surface. But you don't have to. Also apply the same ratio as the image below.
Use a plastic to spread it evenly (glove etc.)