You need a thin sheet of lead or pewter, but steel probably can work without being illegal / environmentally damaging. Preferably the metal sheet should be close to dark grey and not shiny.

Using a knife you must deeply scratch the metal to engrave the names or a sufficient description of who you will curse on it in sentences like "I CURSE ALAN SMITHEE, JAN NOWAK, (…) FOR THE TRANSGRESSION OF (…) Make sure the lettering is small enough. If the different people did different things you have to write it too.

Make sure to make your demands be in-between sycophantic and entitled. The chthonic powers like brazenness and high emotion, but at the same time to be addressed with respect and the praise reserved for a god. You are not addressing something that empathizes with you (that is a grave mistake). You are addressing something that has power and enjoys it and it likes to do little favors for humans as a way of enjoying that power.

Then, you need to fold it once with a hammer, and then drive a nail through the tablet.

And finally, drop the tablet in a place physically deep within the Earth, in a place without water to corrode it and where it will not be disturbed. The drop part is important: it has to be in some place “deep” enough to be unreachable once dropped. It has to fall into inescapable darkness. You should not look down or look back.