chamomile extract from Voyageur Soap & Candle), and I will indicate if they are liquid (like the liquid green tea extract I get from the Personal Formulator).
To use a powdered extract, I dissolve my powder in a titch of heated water (which you can remove from your heated water phase or just add at the cool down phase point) - about 10 ml for 0.5 grams of powder - and stir until dissolved. I add it to my cool down phase at the appropriate temperature (45˚C or less). You can add it as a powder into the cool down phase, but I really prefer to dissolve it then add it.
You can substitute some of the extracts with essential oils, but you will find the smell might be overpowering in some products, especially in leave on products for the face. It's up to you to try it and see what you think.
There are other types of extracts available, mainly as liquids or CO2 extracts. I have a few liquids - Mulitfruit BSC, acai biofunctional liquid, Vital Hair & Scalp Complex, and the aforementioned green tea extract - and I find them very easy to use. I haven't used the CO2 extracts because I can't get them locally. The liquids are easier to incorporate into the proper phase (mostly the cool down phase), but they tend to be more expensive than the powders.