Nanoparticles are made several different ways!! Some nanomaterials are made by etching down larger pieces of material until they are nano-sized. But most nanoparticles are made in a wet Chemistry reaction that takes salts of the material, like gold salts, and heats them up until they start growing into nanoparticles.
Nanoparticles are tiny objects. Imagine if we could cut up a meter into 1 billion pieces… a nanoparticle is only as big as one of these pieces! Since nanoparticles are so small, most of the particle is surface… in other words the particles have high surface area. This high surface makes nanoparticles more reactive than bulk materials. That is why nanoparticles can be used for special applications that larger materials can’t. There are nanoparticles made out of platinum in your car that make the exhaust cleaner for the environment!
My answer is more basic, as we are not experts in nanoparticles, we just need them to be as small as possible.
What I use is basically very small particles, that are a few nanometres in size. We make those by putting larger ones in a liquid, and mixing really hard for a few hours, until they have broken down from a few millimetres to a few nanometres.