Basil: pick off leaves individually or prune all around the plant (never taking more than one-third of entire growth at once time, then waiting for the plant to regrow before pruning again) and use either individual leaves or sprigs for cooking. Stems, of course, should be removed before serving the dish. Harvest regularly to encourage more growth. Some people say the flavour changes once the plant flowers, but I've never found this to be the case - mine flower from tiny and never stop! The flowers are edible. The plant will die as the cold weather approaches. That's normal. It's an annual.

Sage: as for basil. The flowers are edible and do not need to be removed.

Oregano: as for basil. The flowers are edible and do not need to be removed. This plant can spread quite quickly, so keep an eye on it.

Parsley: trim off all over the plant if desired, or just pick stems, as far down as you like. I like to take the lower stems and leaves, encourage more top growth. Once a flower stem appears (in its second year), the flavour becomes more coarse, but if you let it go to seed, you'll get lots more plants next year. It's a biennial.

Coriander/cilantro: be quick if it's hot in your area! This plant will bolt to seed very quickly, and in a hot climate may live out its entire life-cycle in a mere 2 weeks. Once the leaves begin to turn lacy/feathery, and begin to produce a flower stem, that's your warning. Again, pick off individual leaves, or cut whole stems. The feathery leaves are edible, but of much coarser flavour than the 'real' leaves. There's not a lot you can do to stop it going to seed. Let it happen. The seeds are edible when fully ripe, and some will inevitably drop to germinate and start new plants. Best to sow seeds about every 2-3 week to get successive crops. It's a short-lived annual.

Honestly, there's no talent or special knowledge required for harvesting or pruning. A lot of it is just plain commonsense, and no special knowledge is required. If you want a leaf or a sprig, take one! Take 2. Take 10. Just remember the rule of thumb - take only one-third of total growth at any one time - regardless of how big the plant is. Doesn't matter where you cut the stems, really, unless you have an eye for aesthetics!

If you want to harvest your herbs for medicinal purposes, there are optimum times to get the best medicinal properties from them, but really, it isn't critical.

The only thing I would add is that if your sage is a baby plant, I would harvest much less than 1/3. Sage is a perennial and I would would only harvest it very lightly (maybe a few leaves, pinched terminals maybe) if it is very young. If you do this, your plant will be more robust next year with larger possible harvests. From seed, it's usually around 3 years for perennials to get full-sized.

Oregano is a perennial too but is far faster grower than sage and can tolerate more clipping at a younger age. Still, if it is tiny, give it time to establish itself before any heavy pruning.

FataMorgana

Thanks to both of you for the info on cutting herbs. You gave me exactly what I needed.