It depends on the plant, and the part of the plant you're using, and what purpose you're using it for AND what you're infusing it into. There are a lot of variables.
Being a beginner, I would use only dried plant material.
Yes, you can use peppermint leaves, but you're not going to get a lot of scent from them that way. If you use this infused oil in conjunction with peppermint essential oil, it can be a nice way to layer the fragrance giving you more staying power.
I usually fill a glass quart jar approximately 1/3 to 1/2 full of cut, dried plant material. Every plant is different in terms of weight, so again, this is another variable. I don't like using powdered plant material for this purpose because it just sits in the bottom of the jar, not allowing the oil to really extract all of the properties of the plant. It becomes a thick sludge. Just roughly crumble your dry plant material up with your hands up to half of the jar full, then cover with the oil you are using to within 1" of the rim of the jar. I usually use olive oil, but there are times when I use different oils, even castor. Put a lid on it. *Always* use glass, and always store your infused oil in glass.
Get a pot that's at least as deep as the rim of your jar & put some tea towels or rags in the bottom to prevent the bottom of your jar from touching the hot metal & overheating your oil, which will cook your plant material, which you don't want. Put your jar with the lid on in the pot on top of those towels, then fill the pot to within a couple of inches of the rim of your jar. Bring it up to a steaming temperature, not a rolling boil, then turn down to low. For peppermint, allow this to infuse for at least an hour. I usually do 2-4 hours, depending on the plant I am infusing. Water will be evaporating, so replace with HOT water whenever necessary. Cool or cold water can cause your hot jar to shatter. Pour that HOT water in to the pot gradually back up to the same level it was each time you do this.
Shut the heat off when done, leaving the jar in there until the water completely cools. Remove from the water, dry off & label with the date, the plant you used & the base oil you infused into eg. olive oil. DO NOT STRAIN. Let that plant material sit in there for a good month before using. Give it a good shake every week. I prefer leaving this to finish infusing for several months. You can then strain if you choose to. Many plants I do not strain our at all, such as cottonwood buds. I just pour the oil off.
Store in a *cool* cupboard, closet or room to maximize shelf life.
This is NOT the 'double boiler' method, as I see many people incorrectly calling it. It is the water bath method. They are 2 completely different methods.
FYI in some people's opinions, this may be 'overkill'. That's their opinion. In my experience, this is how I get excellent oil infusions & have been doing so for decades. I have also found that by leaving the plant material in the oil, the infused oils remain shelf stable for longer due to their natural antioxidant properties, antibacterial antifungal etc properties.
Good luck