Right now, so many things are prime for harvesting (fancy talk for picking). So many plants are best used medicinally when they are flowering. You need to study and learn the particulars for each plant and respect that some things provided by nature can be hazardous when not used properly.
The plant I have here is prunella. I know that I can safely use all the aerial parts, or what I see above the dirt.
All I did was snip it with some scissors, flowers, stalks and leaves.
Now I need to actually dry the plants.
I prefer to use the large screens off my windows. I sandwich the herbs between the 2 screens and allow them to dry in the shade on a warm, sunny day. You generally don't want to put them in direct sun or where they can get wet. Wet generally defeats the purpose of drying, you know. The direct sun can over heat or 'cook' the herbs and they will not be as useful.
Also make sure that there is plenty of air able to circulate. You don't want to put the plant material out on a solid surface covered by a screen. You want to screens for circulation.
Now the herbs just need to sit and dry. Depending upon your weather this could take hours or a few days. I'll check these after about 12 hrs and then every few hours to see if they are ready.
They shouldn't look burned or crispy, but be light and dried as the moisture is all that is missing from the plant material.When they are dried, use them or store them in a glass jar with a lid.
Make sure they are labelled with the herb, date and where you gathered from, if needed.
This is the same prunella, and it is still in rather large pieces. I'm planning on using it right away so I'll cut into smaller pieces based on that use.
Now your herbs are ready to be used as dried for teas, tinctures or salves. Enjoy!