Metheglins have probably been around almost as long as traditional meads. By definition metheglins are traditional meads made with herbs and/or spices for flavoring, preserving, medicinal, or other purposes. I imagine that the first meads being naturally fermented might have tasted like a honey solvent.
It wouldn't take too long for people to begin adding herbs or spices to make the mead palatable. Even though honey has anti-bacterial properties, early people wouldn't have known this. They might have added herbs or spices in an effort to preserve the mead as long as possible. Some of the herbs may have actually had a preservative effect which allowed for larger batches that stayed drinkable longer. Since there were no "doctors" in those days, the local healers would have added bitter tonics and medicinal herbs into the mead to make them easier to take. Adding medicinal herbs to make metheglins was also a way to preserve the herbs for use in the winter months.
Whatever the original reasons for adding herbs and spices to meads, metheglins have been with us ever since. The word metheglin is derived from the Welsh word "Meddyglyn" which comes from the Latin "medicus", a doctor and means "medicinal liquor." Metheglin is described in the diaries of 17th century diarist Samuel Pepys as "strong in the superlative, if taken immoderately, doth stupefy more than any other liquor and keeps a humming in the brain". He also wrote "the naturallest drink of the Country, being made of the decoction of water and honey".
There are several ways of adding herbs or spices to your mead. The same general rules of adding any plant materials apply. Dried plants are more concentrated than fresh and will require you to use less for the same flavor. Don't use any plant you are unsure of. Many medicines today originated from plants and just because they are not in the form of a pill or syrup, don't assume herbs are safe. Use a reputable herb guide when you are not sure if a plant is safe to use.
If you've never used an herb or spice in a mead, try making 1 gallon batches with different strengths or quantities until you find the one you like. Then you can scale it up to 5 gallons. Many herbs or spices will overpower metheglins and it's difficult to reduce the effect once you've added the herb or spice. There are plenty of great recipes on the internet and Ken Schramm's book The Compleat Meadmaker: Home Production of Honey Wine from Your First Batch to Award-Winning Fruit and Herb Variations.
The simplest method would be to add about a cup of fresh herbs (or a tablespoon or two of dried) into your fermenting mead. The heat and alcohol from fermentation will take care of the rest. Eventually the plant material will sink to the bottom where you can rack the mead leaving it behind. If some of the spices or herbs are floating in the mead, you may need to either rack from under them, or use a strainer. I don't think that leaving the plant material for a week or two would hurt, but much more than that and the organic material may start to break down or rot.
Many meadmakers put their herbs and spices into a "tea bag". To do this, add the spices to a coffee filter or piece of cheesecloth and bring the open end together and close either with string, tie-wrap, or staple with a string attached. Leave the string hanging out of the fermenter so you can have access to it. You can put a rubber stopper into the neck of a carboy with the string hanging out if you like. I have added a sanitized marble or other weight into the bag to allow it to sink into the metheglin. Once it has been in the mead for a day or two, dip the bag in and out of the fermenting mead several times to help extraction, just like you would a tea bag in hot water. The tea bag method is easy. It allows you to quickly remove the herbs or spices without having to rack or strain them out of the mead when you feel the mead tastes right. Be careful with fresh plants that may harbor wild yeast and/or bacteria. You will find that dried herbs and spices generally have less molds, fungi, wild yeast and bacteria than fresh plant material.
Another method is to make your own extracts from your favorite herbs, spices or flowers. To do this, find a suitable supply of small jars such as baby food jars with tight fitting lids. Add a few tablespoons of a single herb to each jar and fill with ever-clear or cheap vodka. The everclear will extract more than the vodka because it contains less water. Leave the jars in an area where they won't be bothered and be sure to agitate them at least daily for a few weeks to a couple of months. It will vary as to what you are trying to extract. To do a pilot test with these extracts, try adding a milliliter at a time to an ounce of traditional mead. When you get the flavor you like, you can scale-up the quantities for 5 gallons of metheglin.
You can use just about any herb, spice or flower to make your metheglins. But be careful, if you wouldn't drink a tea made from it, don't add it to your mead. Some of the additives you can use include ginger, vanilla, citrus peels, coriander, cloves, cinnamon, lemongrass, peppers or the seeds of peppers, nutmeg, oregano, basil, hops, chamomile, chocolate, coffee, rose petals, or nuts of various kinds. Just about anything may be added. Many meadmakers use a combination of these herbs and spices to make wonderfully complex meads. Just be sure to keep good notes on the quantities and methods you use when adding these spices and herbs so that you can repeat the recipe again.
Vanilla is a popular spice for metheglins. It smooths out the harsh edges of some meads and makes any mead seem softer and more mellow. Buy the best vanilla beans you can afford. You can either add the beans to the mead or make your own extract. When adding the beans, split the bean lengthwise and use the tip of a knife to scrape all the pulp (where most of the flavor is) into the fermenting must. Soaking time will vary as will the number of vanilla beans you use, depending on their freshness and quality. Consult a few recipes for guidelines. You can start with one vanilla bean and add more, whereas it's impossible to take away the vanilla flavor that ten beans give to a mead. Use your own sense of taste and judgment, when it seems right, pull them out. Many say you can add the beans to a small container of sugar to make vanilla-sugar for flavoring deserts.
There are many quality extracts on the market. The problem with extracts is that they sometimes make metheglins taste artificial. If in doubt, do a pilot-test with small amounts and see how it tastes. Scale it up if you like the flavor or use fresh beans if you don't. You can make your own by adding several beans, split and scraped, in a small jar with a tight fitting lid, filled with vodka or everclear. Give the extract several weeks to a month or more to work and shake it often. Add the extract directly to the mead during fermentation or prior to bottling.
Many of the herbs or spices may be added in the bottling bucket rather than during fermentation. I would say that strong extracts that you know will blend well into a mead you enjoy would be fine added to the bottling bucket or secondary fermenter prior to bottling. Some of the herbs which require extraction of the essences will take much longer and require the alcohol produced during fermentation to help extract all the flavors you are looking for. Experience and lots of practice will help you decide how long to keep the spices and herbs in your meads. Be sure to consult proven recipes and take good notes for repeatability. If you find a recipe for metheglins you like and one that wins awards, be sure to pass it along to the meadmaking community. Everyone is here to help you make the best metheglins you possibly can and you can return the favor after you gain experience and start making award winning mead.