First, understand that many grimoires give elaborate instructions in obtaining ingredients for incenses, oils and spells. Pay no attention to these things.
Things like “collect the Vervain at the third hour of the Full Moon when Saturn is in Pisces, and only collect from the hallowed ground of a churchyard” would not make the Vervain any stronger. This long and drawn out instruction is given to sort the dabbler from the serious Witch. The Witch Proper knows to pay attention to the first part of the instruction, and ignore the latter.
It matters not where you obtain your ingredients. Obtain them any way you can in the most convenient way you can.
If you do get them from the wild, or from your garden, leave enough so that the plant can recover unless it is the final harvest of the season. Always thank the spirits of the land for their gift. A little bit of something as a gift is always good, and can be bread, wine, or something of you such as a drop of blood.
If you do harvest the herbs yourself, make sure you dry them completely before using them. Hanging them in the window in the sun is the best way that I’ve found.
Always know your herbs. If they are poisonous, use rubber gloves! Be mindful of the amount of the darker herbs that you use and their effect. Too much can be dangerous and sometimes even fatal.
Oils can be made if you wish to go through the trouble, but are found with a little bit of legwork much easier. The same goes for oils as with herbs. Know your oils and their effects before you use them. Again, gloves are appropriate for the darker oils.
Keep your herbs in clean jars with lids that seal. You can also use Zip-Loc bags. Keep them in a cool, dry location. Most will stay well for a good period of time. Label them well.
Oils should be kept in jars that seal as well. Keep them in a cool dark place. Make sure you mark them well with a marker that is permanent! Usually the best way is to label the top of the lid as oil tends to get onto the side of the container with use and will destroy any label.
Oils will stay good for about one year after the container is opened. After that they may spoil and will not work well.