It starts the way Nu-Nu describes it, as a dominance establishment. Once that is out of the way, it can be weeks, or even years before the net stage. The male will pay more attention to the female's anal fin, and the female will get plump. When spawning is nearer, the two will begin to obsessively clean an area of rock or some other surface with both mouths and fins. This will be most likely very near whatever they have hosted, if anything. It will be in their territory, though.
When spawning is imminent, the female's ovipositor will become visible. Actual egg laying will probably be after dark. The female will lay her eggs and the male will follow to fertilize. They will guard the eggs until they hatch.
Once they hatch, if you want them to survive, they will need to be removed. The odds of clownfish fry surviving in a display tank are very slim.
There are some great articles and even entire books dedicated to breeding clownfish. It's a really fun read! I eventually hope to give it a try myself.
Hope this helps!
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef