Nearly always by attacking the bottom slat, the lowest one, which is the only one that can be gripped from outside. There are two usual ways:
- Lifting it. Something is slid underneath, it is levered and the shutter rises up the guides far enough to slip through or reach the window.
- Bending it. The slat is pushed in the middle until it curves and comes out of the guides.
A slat is a long, thin strip. Push it in the middle and it gives; pull it from below and it rises. And there is usually no need to break anything noisy.
It happens above all on windows facing back patios, screened terraces or quiet communal areas, and in homes that stay empty for a while. That is why we work on exactly that slat: see shutter security.