Grind the entire cannabis plant or just the buds or a little bit of both it's just a matter of preference. Do not grind the cannabis to fine or it will end up going thru your strainer and get in the oil.
Combine your marijuana and your oil in your slow cooker or double boiler and heat the two together on low or warm for a few hours. This will allow the marijuana to go thru the decarboxylation process. You can cook the batch on low for about 4 to 6 hours if you're using a double boiler or slow cooker stirring the contents occasionally. If you're using a double boiler then use a low setting for 6 to 8 hours. You can use a simple saucepan on low and stir frequently for about 3 hours. Just know using a saucepan gives you a much greater chance of scorching the mixture which will destroy the THC and make your oil crap. You can use a small amount of water to add to the mixture to help avoid burning. Make sure that the temperature of the oil does not exceed 245 Degrees Fahrenheit.
Strain the oil and store it. When you strain the oil do not squeeze the cheesecloth this does not help and it will add more chlorophyll to your canna oil.