Are you talking about harvesting for drying? Or for using fresh? If for fresh, you can just pick off leaves for cooking... just noting that most fresh herbs aren't as strong as dried...

for drying, I don't know about any sites, but you can pic 'branches' of oregano, parsley, though dried parsley isn't that great, and the rest and then tie together and hang to dry. chives... well, you can just dry them on plates. It's fairly easy, a 'no fault' kind of thing... just don't harvest more than a 3rd of the plant to keep it healthy.

Really, you can harvest these herbs at any time, provided you don't remove more than one-third of total growth at any one time, and allow the plant to regrow before harvesting again. So if a plant has 3 leaves or sprigs, you can take one of them.

It's usually recommended to harvest in the morning, after the dew has dried from the leaves. An exception is lemon balm, which is best harvested in the afternoons according to some (but don't ask me why!).

There are optimal times to harvest if the plant is to be used for medicinal purposes, or dried for future use. This especially applies to herbs where the roots, flowers, berries, bark etc are used, but it does apply in many cases with leaves, too. The medicinal properties may be stronger when the plant is in flower, or just before it flowers, or just after, or when it's 3yo, or whatever. There are really very few hard and fast rules, simply because there are so many herbs (around 20000).

You can find out by googling for something like 'harvest(ing) mint'.

However, this might get you started:

Oregano - Pick leaves whenever available to use fresh. Begin harvesting when plants are about 10-12 cm high. They can be dried or frozen, or put in oil or vinegar. Flavour is best just after the flower buds form, usually mid-summer.

Parsley - Leaves can be harvested as soon as the plants reach about 15cm tall and produce leaves with three segments. Lower, outside, leaves should be harvested first, leaving the centre top leaves to mature. First season of growth has the best flavour. Roots may be harvested once plants are mature.

Coriander (which you call cilantro) - Leaves should be harvested as soon as they are large enough for use. They may be frozen, but do not dry well. To harvest seeds, let the plants grow until the first set of seeds begins to turn brown and dries enough to crack when pinched.

Sage - Harvest leaves at any time when the plants are flourishing to be used fresh but before the flowering shoots develop in summer if they are to be dried. Take the stem tips with several sets of leaves. Cut in the morning after the dew has dried but before the heat of the afternoon. Snip off and dry the whole flowering stems of older plants for decorative or culinary use.

Rosemary - Leaves can be harvested at any time, preferably before flowering, in summer, on sprigs about 10cm long. Never take more than 20% of the plant at any one time. They dry well but care must be taken not to use excessive or prolonged heat during the drying process as the oil is very volatile. Freezing is not recommended.

Garlic chives - Cut off clumps of young leaves at ground level. Chop and freeze the tender shoots for winter use. Stop harvesting in early autumn to avoid weakening the plants.

Thyme - Leaves can be harvested for fresh use throughout the summer but the flavour is best just before flowering. To dry, cut the stems just as the flowers start to open and hang in small bunches. Harvest sparingly the first year.

Lemon mint - (as with most mints.) Pick leaves as required throughout the growing season to use fresh, or in summer before flowering if for drying (cut back 3-6cm above the soil line). The edible flowers can be harvested as required.