Nice sturdy case closer (there are plenty of cheap flimsy ones about). You can snap many a case closed after battery changes etc. with strong fingers, but stubborn ones can be a real nuisance.
This case closer is heavy and all-metal construction apart from the interchangeable dies, which are necessarily plastic so that they don't scratch or damage the watch case or glass. When you first take it out of the box, it's in the closed position so turn the small key on the side to unlock it. Then you need to choose one of the concave dies to fit the front of your watch - find one where the rim of the die will press on the outer metal case of your watch front, not on the glass itself, then place your chosen die on the lower part of the press (just push it down onto the spoke). Then choose a flat (non-concave) die which will cover all or most of the back plate of your watch. Push that up onto the upper spoke of the press. Carefully align your watch on the press so that the glass fits into the dent/concave and the loose case back is right in position* and push the main press lever down. Job done. I've added a photo at the top of the page showing it set up ready to press.
*One tip: when you place the loose case back onto the watch case, ready to press, always check whether there's a small notch in the rim of the case back - there often is - to accommodate the winder stem. If so, be sure to line it up over the winder stem.
You might find with today's bigger watches you'll wish there were some larger sized dies in this kit (biggest provided is 40mm) but there are a few places on-line where you can buy those separately.