Different people have slightly different techniques, and you may want to experiment a bit to find your own "sweet spot".
Suggest you start with a grind close to drip. Water to coffee ratio about 16:1
The process works best when using the full capacity of the brewer, so that's a lot of coffee in the one you have - you can get away with a lower volume, but easier if you buy ALL the Cona sizes then you don't have to compromise
Fill the bottom bowl fairly close to the top - say about 3-4 cm below the neck, but it isn't too critical.
The bowl will be OK directly on the gas hob - provided you don't "ding" it on the supports.
Heat the water.
At this point opinions differ - some folk put the ground coffee in the top straight away, and fit the top bowl - this is what Cona suggest. Others wait to fit the bowl until the water is pretty close to boiling. Others wait until the water has migrated to the top before adding the coffee. Whichever technique you use,
do not disturb the filter rod at all after adding the coffee. If you get even small amounts of grounds under the seat of the filter, it won't filter properly.
The principle is that the pressure created in the bottom bowl forces the water up into the top. It doesn't empty completely. At this point drop the heat to the point where it just maintains enough pressure to keep the water in the top and gently swirling around with the coffee.
You may want to give it a careful stir, but don't move the rod.
At the point you want the brew to stop - opinions vary greatly - say between 2 and 4 minutes - drop the heat.
As the bottom bowl cools,a partial vacuum forms and sucks the coffee back down. The glass filter is designed to let the coffee pass but hold back the grounds.
It's simpler than it sounds, but glass filter rods can be a bit finicky, and you do need a good consistent grind (not too much dust) for it to work properly.
It's worth the effort because in the opinion of a lot of people - including me - it is the best way to brew coffee.